s3/loadparm: ensure default service options are not changed

s3/loadparm: ensure default service options are not changed

Rename sDefault to _sDefault and make it const. sDefault is make a copy
of _sDefault in in the initialisation function lp_load_ex().

As we may end up in setup_lp_context() without going through
lp_load_ex(), sDefault may still be uninitialized at that point, so I'm
initializing lp_ctx->sDefault from _sDefault.

Bug: https://bugzilla.samba.org/show_bug.cgi?id=13051

Signed-off-by: Ralph Boehme <slow@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>