mail/milter-greylist: minor fixes

• Use LIB_DEPS for curl, spf and geoip
• Use ${PORTSDIR} instead of {.CURDIR} for bsd.milter.mk location
• Do not bump PORTREVISION as all of curl, spf and geoip are disabled by defaul

PR: 193993
Submitted by: Mikhail Tsatsenko <m.tsatsenko@gmail.com>
Approved by: mentors (implicit)