pgsql: Further fixes for per-tablespace options patch.

Log Message:
-----------
Further fixes for per-tablespace options patch.

Add missing varlena header to TableSpaceOpts structure.  And, per
Tom Lane, instead of calling tablespace_reloptions in CacheMemoryContext,
call it in the caller's memory context and copy the value over
afterwards, to reduce the chances of a session-lifetime memory leak.

Modified Files:
--------------
    pgsql/src/backend/utils/cache:
        spccache.c (r1.3 -> r1.4)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/cache/spccache.c?r1=1.3&r2=1.4)
    pgsql/src/include/commands:
        tablespace.h (r1.22 -> r1.23)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/commands/tablespace.h?r1=1.22&r2=1.23)