Support `vCont` packets with '0' thread-id in single-threaded operation

[rayc345]

When using probe-rs with Segger Embedded Studio to debug a MCU, gdbstub seems not able to recognize some commands. "Client sent an unexpected packet. This should never happen! Please re-run with log trace-level logging enabled and file an issue at https://github.com/daniel5151/gdbstub/issues" It seems the issue lines in '$vCont;s:0#22' In resume.rs:206 seems to be the origin. GDBstub uses the latest version from GitHub repository. Here is the GDB comm:

write: $qSupported:vContSupported+#6c
read: $PacketSize=1000;vContSupported+;multiprocess+;QStartNoAckMode+;hwbreak+;qXfer:features:read+;qXfer:memory-map:read+#72
write: $vMustReplyEmpty#3a
read: $#00
write: $QStartNoAckMode#b0
read: $OK#9a
write: $!#21
read: $#00
write: $qXfer:memory-map:read::0,ffe#1b
read: $m<?xml version="1.0"?>[0a]<!DOCTYPE memory-map PUBLIC "+//IDN gnu.org//DTD GDB Memory Map V1.0//EN" "http://sourceware.org/gdb/gdb-memory-map.dtd">[0a]<memory-map>[0a]<memory type="ram" start="0x0" length="0x20000"/>\n<memory type="ram" start="0x80000" length="0x20000"/>\n<memory type="rom" start="0x80000000" length="0x100000"/>\n<memory type="ram" start="0xf0400000" length="0x8000"/>\n</memory-map>#55
write: $qXfer:memory-map:read::188,ffe#8c
read: $l#6c
write: $qXfer:features:read:target.xml:0,ffe#7c
read: $m<?xml version="1.0"?>[0a]        <!DOCTYPE target SYSTEM "gdb-target.dtd">[0a]        <target version="1.0">[0a]        <architecture>riscv:rv32</architecture><feature name='org.gnu.gdb.riscv.cpu'><reg name='x0' bitsize='32' type='uint32'/><reg name='x1' bitsize='32' type='uint32'/><reg name='x2' bitsize='32' type='uint32'/><reg name='x3' bitsize='32' type='uint32'/><reg name='x4' bitsize='32' type='uint32'/><reg name='x5' bitsize='32' type='uint32'/><reg name='x6' bitsize='32' type='uint32'/><reg name='x7' bitsize='32' type='uint32'/><reg name='x8' bitsize='32' type='uint32'/><reg name='x9' bitsize='32' type='uint32'/><reg name='x10' bitsize='32' type='uint32'/><reg name='x11' bitsize='32' type='uint32'/><reg name='x12' bitsize='32' type='uint32'/><reg name='x13' bitsize='32' type='uint32'/><reg name='x14' bitsize='32' type='uint32'/><reg name='x15' bitsize='32' type='uint32'/><reg name='x16' bitsize='32' type='uint32'/><reg name='x17' bitsize='32' type='uint32'/><reg name='x18' bitsize='32' type='uint32'/><reg name='x19' bitsize='32' type='uint32'/><reg name='x20' bitsize='32' type='uint32'/><reg name='x21' bitsize='32' type='uint32'/><reg name='x22' bitsize='32' type='uint32'/><reg name='x23' bitsize='32' type='uint32'/><reg name='x24' bitsize='32' type='uint32'/><reg name='x25' bitsize='32' type='uint32'/><reg name='x26' bitsize='32' type='uint32'/><reg name='x27' bitsize='32' type='uint32'/><reg name='x28' bitsize='32' type='uint32'/><reg name='x29' bitsize='32' type='uint32'/><reg name='x30' bitsize='32' type='uint32'/><reg name='x31' bitsize='32' type='uint32'/><reg name='pc' bitsize='32' type='code_ptr'/><reg name='pc' bitsize='32' type='code_ptr'/></feature></target>#c9
write: $qXfer:features:read:target.xml:69f,ffe#21
read: $l#6c
write: $g#67
read: $0000000000000000a0ffffff0000000000000000000000000000000000000000000000000000000003000000a800000000000000000000008913000002000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000#c8
write: $mfffffff8,c#fe
read: $E7a#dd
write: $m0,100#5a
read: $E7a#dd
write: $m0,4#fd
read: $E7a#dd
write: $P0=00000000#3d
read: $OK#9a
write: $P1=00000000#3e
read: $OK#9a
write: $P2=00000000#3f
read: $OK#9a
write: $P3=00000000#40
read: $OK#9a
write: $P4=00000000#41
read: $OK#9a
write: $P5=00000000#42
read: $OK#9a
write: $P6=00000000#43
read: $OK#9a
write: $P7=00000000#44
read: $OK#9a
write: $P8=00000000#45
read: $OK#9a
write: $P9=00000000#46
read: $OK#9a
write: $Pa=00000000#6e
read: $OK#9a
write: $Pb=00000000#6f
read: $OK#9a
write: $Pc=00000000#70
read: $OK#9a
write: $Pd=00000000#71
read: $OK#9a
write: $Pe=00000000#72
read: $OK#9a
write: $Pf=00000000#73
read: $OK#9a
write: $P10=00000000#6e
read: $OK#9a
write: $P11=00000000#6f
read: $OK#9a
write: $P12=00000000#70
read: $OK#9a
write: $P13=00000000#71
read: $OK#9a
write: $P14=00000000#72
read: $OK#9a
write: $P15=00000000#73
read: $OK#9a
write: $P16=00000000#74
read: $OK#9a
write: $P17=00000000#75
read: $OK#9a
write: $P18=00000000#76
read: $OK#9a
write: $P19=00000000#77
read: $OK#9a
write: $P1a=00000000#9f
read: $OK#9a
write: $P1b=00000000#a0
read: $OK#9a
write: $P1c=00000000#a1
read: $OK#9a
write: $P1d=00000000#a2
read: $OK#9a
write: $P1e=00000000#a3
read: $OK#9a
write: $P1f=00000000#a4
read: $OK#9a
write: $P20=00000000#6f
read: $OK#9a
write: $vCont;s:0#22