cellRtc: implement remaining parsers, improve formatters

[capriots]

• Full implementation of asctime() and RFC2822 parsers
• cellRtcParseRfc3339():
  • Moved digit() out of the function, I'm using it for the other parsers as well
• cellRtcFormatRfc2822() + cellRtcFormatRfc3339():
  • Removed wrong pUtc->tick == 0 check, this is actually a nullptr check on LLE
  • Replaced snpintf() with simple num + '0'
  • Simplified time zone formatting
• Set log level to notice

[Megamouse]

I'll probably review it tomorrow or so

[Megamouse]

Haven't had time to review cellRtcParseDateTime yet. Maybe tomorrow