This patch updates the DDL for contrib/pgcrypto to create all

This patch updates the DDL for contrib/pgcrypto to create all
functions as STRICT, and all functions except gen_salt() as IMMUTABLE.
gen_salt() is VOLATILE.

Although the functions are now STRICT, I left their PG_ARGISNULL()
checks in place as a protective measure for users who install the
new code but use old (non-STRICT) catalog entries (e.g., restored
from a dump).  Per recent discussion in pgsql-hackers.

Patch from Michael Fuhr.