s3:param: split out loadparm_s3_global_substitution from lp_string()

s3:param: split out loadparm_s3_global_substitution from lp_string()

The idea is to get rid of the global state that's
currently used for the substitution in lp_string().

In the end all callers need to pass an explicit
const struct loadparm_substitution *sub_ctx,
which contains all relevant information
for the substitution. At that point lp_string() can be removed.

For now we provide loadparm_s3_global_substitution()
for callers to keep the current bahavior unchanged.

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Ralph Boehme <slow@samba.org>