fileio: port read_file_full() to use connect_unix_path()

fileio: port read_file_full() to use connect_unix_path()

This way we can connect correctly to any AF_UNIX socket in the file
system, and even save some code. Yay!

This also adds some test code for this, that ensures read_file_full()
works correctly for AF_UNIX sockets that violate the 108 char limit.

Supporting sockets like this kinda matters I think, for the simple
reason that apps want to build socket paths via XDG_RUNTIME_DIR and
suchlike, and we should be able to connect to them, even via
non-normalized paths.