MEDIUM: config: move all unix-specific bind keywords to proto_uxst.c

MEDIUM: config: move all unix-specific bind keywords to proto_uxst.c

The "mode", "uid", "gid", "user" and "group" bind options were moved to
proto_uxst as they are unix-specific.

Note that previous versions had a bug here, only the last listener was
updated with the specified settings. However, it almost never happens
that bind lines contain multiple UNIX socket paths so this is not that
much of a problem anyway.