www/miniflux: Remove nop WANT_PGSQL=client

In 8f75e04cb888e86484843db9dca00e739e346e19 the pgsql dependency
was dropped intentionally but WANT_PGSQL was left in place but it
has no effect now.

Fix the PGSQL_SERVER option by adding the required USES=pgsql for
WANT_PGSQL.

While here also fix opt_VARS usage. Unlike opt_USE it does not
support separating words with ,

Reported by: portscan (WANT_PGSQL without USES=pgsql)