networkd: monopolize in_addr utility functions in shared/in-addr-util.h

networkd: monopolize in_addr utility functions in shared/in-addr-util.h

Primarily, this means we get rid of net_parse_inaddr(), and replace it
everywhere with in_addr_from_string() and in_addr_from_string_auto().
These functions do not clobber the callers arguments on failure, which
is more close to our usual coding style.