Debugging binaries within another sysroot on newer kernels

[oxr463]

We used proot and gdb to debug binaries within another sysroot. While it worked with the master branch and with an older kernel it doesn't work with a more recent kernel and/or this patch.

You can reproduce it with this command:

$ proot -R / gdb /bin/true
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Reading symbols from /bin/true...(no debugging symbols found)...done.
(gdb) run
Starting program: /bin/true 
warning: linux_check_ptrace_features: failed to kill child

Program received signal SIGSEGV, Segmentation fault.
0x0000000000000000 in ?? ()
(gdb) bt
#0  0x0000000000000000 in ?? ()
#1  0x0000000000000003 in ?? ()
#2  0x00007fffffffcffb in ?? ()
#3  0x00007fffffffd005 in ?? ()
#4  0x00007fffffffd008 in ?? ()
#5  0x0000000000000000 in ?? ()

With an older kernel and the master branch it works fine and when the application throws a segmentation fault we can catch it in gdb.

With a more recent kernel and the master branch we have to use PROOT_NO_SECCOMP=1 but gdb freezes when the application terminates. With a more recent kernel and this PR we get a segmentation fault as documented above.

Originally posted by @walterbrebels in https://github.com/proot-me/proot/pull/130#issuecomment-344664485