kernel: Add lpathconf(2) syscall.

It's like pathconf(2) but doesn't follow symlinks.

Manual page changes taken from FreeBSD.
kernel/rmdir: Return EBUSY if the dir is a mount point for a mounted fs.

See: http://pubs.opengroup.org/onlinepubs/009695399/functions/rmdir.html

It was already documented like that in the manual page.

While here, also document that we return EINVAL if the last component of
the path is "." or ".." (taken from FreeBSD).

Reported-by:   Joris Giovannangeli <joris@giovannangeli.fr>
Dragonfly-bug: <http://bugs.dragonflybsd.org/issues/2543>
pathconf.2: Mention lpathconf() in the ERRORS section.