socket-util: socket_address_parse() should not log errors on its own

Given that socket_address_parse() is mostly a "library" call it
shouldn't log on its own, but leave that to its caller.

This patch removes logging from the call in case IPv6 is not available
but and IPv6 address shall be parsed. Instead a new call
socket_address_parse_and_warn() is introduced which first invokes
socket_address_parse() and then logs if necessary.

This should fix "make check" on ipv6-less kernels:

http://lists.freedesktop.org/archives/systemd-devel/2015-April/031385.html