getsubids: system binary for user's sub*ids

Rename list_subid_ranges to getsubids to provide a system binary to
check the sub*ids of a user. The intention is to provide this binary
with any distribution that includes the subid feature, so that system
administrators can check the subid ranges of a given user.

Finally, add a man page to explain the behaviour of getsubids.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1980780

Signed-off-by: Iker Pedrosa <ipedrosa@redhat.com>

man/Makefile.am

@@ -62,6 +62,7 @@ man_MANS += $(man_nopam)
 endif
 
 man_subids = \
+	man1/getsubids.1 \
 	man1/newgidmap.1 \
 	man1/newuidmap.1 \
 	man5/subgid.5 \
@@ -80,6 +81,7 @@ man_XMANS = \
 	expiry.1.xml \
 	faillog.5.xml \
 	faillog.8.xml \
+	getsubids.1.xml \
 	gpasswd.1.xml \
 	groupadd.8.xml \
 	groupdel.8.xml \