vfs: Allow non-null-terminated UNIX socket pathnames

This patch fix one of the compatibility issues mentioned in #3255. It
allows applications to call bind() or connect() passing an sockaddr_un
structure with a pathname that is not null-terminated.

Some systems did not require pathname in sockaddr_un::sun_path to be
null-terminated, instead the end of the string is determined by the size
of the structure passed as an argument of bind() or connect().

The standard is a bit vague in this matter but suggest that the path
should be null-terminated and the functions bind() and connect() should
be given sizeof(sockaddr_un) as a structure size.
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