Unify s_client/s_server srtp profiles option handling

Add missing guards around STRP-related fields
Remove two unneeded global variables: my 2'cents to #4679
Merge definition and instantiation of srpsrvparm global.

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Ben Kaduk <kaduk@mit.edu>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4908)