Log Message:
Import some improvements from the pkgsrc-wip/dovecot package (by tv@) and
correct a mistake in my previous commit:

- add dependency on zlib
- add option for GNU TLS as an alternative to OpenSSL
- drop SASL option, it's no longer supported
- explicitly disable some options with --disable-*
- "s,/usr/pkg,@PREFIX@," in patch-ab -- I accidentally diffed it after \ 
the SUBST stage
- change SUBST_STAGE from post-patch to pre-configure to avoid this in the future
- bump PKGREVISION for all this

Thanks to tv@.