Make it possible to use empty user name ("-U ''") for mount_smbfs(8).
It's just like "-U guest", except that it actually works, at least
with Samba 4, which seems to return authentication failure for "-U guest".

MFC after:	1 month
Sponsored by:	The FreeBSD Foundation


git-svn-id: svn+ssh://svn.freebsd.org/base/head@271259 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f