utils: add support for "trimmed abstract" sockets

utils: add support for "trimmed abstract" sockets

Right now plymouth listens on an abstract socket with a name
that has a bunch of trailing NUL bytes.  These zeroes uglify
the output of /proc/net/unix among other things.

This commit adds support for a new "trimmed abstract" socket
type, which drops the zeroes.

A subsequent commit will actually change plymouthd to use the
new api.