mount.cifs: if mount user not specified use USER environment variable

mount.cifs: if mount user not specified use USER environment variable

smbfs also would use the USER environment variable if the user was not
specified on the mount command and no credential file specified, and
mount.cifs man page says that we will use this environment variable
(in most cases this will not cause a behavior change, because
we were doing getuid of the current process which will usually be
the same name).

Fixes Samba bug #5934

Acked-by: Jeff Layton