Change knob IMAP_MODULE to MAIL_MODULE (1).
Keep compatibility for IMAP_MODULE, notice warning if usage.
Cosmetic fixes for package list (2).
(1) Anton Yuzhaninov (citrin at rambler-co dot ru)
(2) portlint -a
Update port infrastructure.
Add compiler's debug flag if WITH_DEBUG defined.
Add prefix NGINX_ for port's variables (VARDIR, LOGDIR, TMPDIR) for
prevent conflict with system variables.
Remove STOP_BEFORE_REMOVE and START_AFTER_INSTALL knobs.