s3:smbd make yp cache local.

s3:smbd make yp cache local.

The my_yp_domain variable is just a static cache needed to avoid
making over and over expensive and potentially blocking calls to
yp_get_default_domain().
Instead of keeping this onto the smbd_server_connection struct, just
keep it local to the only function ever using this variable.

This disentagle this function (and a number of calling functions)
from having to pass around smbd_server_connection and thus having
to link against smbd. It also removes a few ifdefs.

Nothing changes from a global/local pov, as the smbd_server_connection
variable passed around is also a global one.

Signed-off-by: Andreas Schneider <asn@samba.org>