param: change snum parameter in special functions to a loadparm_service pointer

Using either an snum or the current service for special functions
depending where it was called is unnecessary and complicates the process
of handling them generically.

idmap parameters now call lpcfg_do_parameter_parametric and are now set
like normal.

Change-Id: I3eca89791274280f9d3c7b987fbd790c16ec7981
Signed-off-by: Garming Sam <garming@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Nadezhda Ivanova <nivanova@samba.org>