vpopmail: build & install from source

[msimerson]

Changes proposed in this pull request:

• mysql support was recently removed from the vpopmail port, build from source instead
• vpopmail: support for QMAIL_EXT
• vpopmail: add TOASTER_VPOPMAIL_CLEAR setting
• vpopmail: create aliasdomains table
• dovecot: update SQL to support valiasdomains
• fixes #481
• remove installs of dialog4ports (deprecated), auto-install portconfig when building from ports
• vpopmail: installed maildrop 3.1.0, workaround for #534
• qmail: set QMD VPOPMAIL_EXT when configured

Checklist:

• [x] docs up-to-date