core: support IEC suffixes for RLIMIT stuff

core: support IEC suffixes for RLIMIT stuff

Let's make things more user-friendly and support for example

  LimitAS=16G

rather than force users to always use LimitAS=16106127360.

The change is relevant for options:

  [Default]Limit{FSIZE,DATA,STACK,CORE,RSS,AS,MEMLOCK,MSGQUEUE}

The patch introduces config_parse_bytes_limit(), it's the same as
config_parse_limit() but uses parse_size() tu support the suffixes.

Addresses: https://github.com/systemd/systemd/issues/1772