sendmail: Move libmilter to its own package

Many things other than sendmail itself use the milter API, for example
mail/opendkim from ports.  Putting the library in its own package means
those applications don't need to depend on FreeBSD-sendmail.

Reviewed by:	manu
Differential Revision:	https://reviews.freebsd.org/D51914

lib/libmilter/Makefile

@@ -1,6 +1,6 @@
 .include <src.opts.mk>
 
-PACKAGE=sendmail
+PACKAGE=lib${LIB}
 SENDMAIL_DIR=${SRCTOP}/contrib/sendmail
 .PATH:	${SENDMAIL_DIR}/libmilter ${SENDMAIL_DIR}/libsm
 

release/packages/ucl/libmilter-all.ucl

@@ -0,0 +1,6 @@
+comment = "Sendmail mail filter (milter) library"
+desc = <<EOD
+The Sendmail milter interface allows a mail transport agent to filter mail
+through a separate process, called a milter.  libmilter provides support
+for applications implementing the milter interface.
+EOD