mail/opensmtpd-extras: Update to 2016-05-18 snapshot

• Update to version 201605182041
• Remove post-patch for -clamav -spamassassin (is now configurable)
• Remove post-patch for -regex (fixed upstream)
• USE_MYSQL to USES= mysql

Reviewed by: gahr (maintainer), adamw
Approved by: gahr (maintainer), adamw
Differential Revision: D6462