MFC r271258:

Make mount_smbfs(8) preserve the "automounted" mount flag.

The issue here is that we have to pass this flag as a string,
in iov, because it doesn't fit in mntflags, which is an int.

Approved by:	re (gjb)
Sponsored by:	The FreeBSD Foundation
MFC r271259:

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".

Approved by:	re (gjb)
Sponsored by:	The FreeBSD Foundation