Check that UNIX socket filenames are not too long.

Check that UNIX socket filenames are not too long.

UNIX socket filenames must fit in a struct sockaddr_un, and typically this
only has about 100 bytes of storage. This is perfectly fine for normal use
of tinc, but this caused failures when running make distcheck, which ends
up creating a rather deep directory structure. With this commit, at least
a proper error message is printed instead of silently truncating the
filename.