check-old-libs: Show information about packages

If pkg is installed, for each old library found to still be present,
check if any installed packages either provide or require the library,
and inform the user.

MFC after:	1 week
Reviewed by:	markj
Differential Revision:	https://reviews.freebsd.org/D53977

Makefile.inc1

@@ -3816,6 +3816,15 @@ check-old-libs: .PHONY
 	while read file; do \
 		if [ -f "${DESTDIR}/$${file}" -o -L "${DESTDIR}/$${file}" ]; then \
 			echo "${DESTDIR}/$${file}"; \
+			if packages=$$(${LOCALBASE:Q}/sbin/pkg -r ${DESTDIR:U/} \
+			    shlib -qP $${file##*/} 2>/dev/null) && \
+			    [ -n "$${packages}" ]; then \
+				echo "    also provided by:" $${packages}; \
+			elif packages=$$(${LOCALBASE:Q}/sbin/pkg -r ${DESTDIR:U/} \
+			    shlib -qR $${file##*/} 2>/dev/null) && \
+			    [ -n "$${packages}" ]; then \
+				echo "    required by:" $${packages}; \
+			fi; \
 		fi; \
 		for ext in debug symbols; do \
 		  if [ -f "${DESTDIR}${DEBUGDIR}/$${file}.$${ext}" ]; then \