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.

MFC after:	2 weeks
Sponsored by:	The FreeBSD Foundation


git-svn-id: svn+ssh://svn.freebsd.org/base/head@271258 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f