core: support <soft:hard> ranges for RLIMIT options

The new parser supports:

 <value>       - specify both limits to the same value
 <soft:hard>   - specify both limits

the size or time specific suffixes are supported, for example

  LimitRTTIME=1sec
  LimitAS=4G:16G

The patch introduces parse_rlimit_range() and rlim type (size, sec,
usec, etc.) specific parsers. No code is duplicated now.

The patch also sync docs for DefaultLimitXXX= and LimitXXX=.

References: https://github.com/systemd/systemd/issues/1769