xbps-src: add consistency check

`xbps-src consistency-check` iterates over all templates and looks for
potentially broken dependencies.

xbps-src

@@ -30,6 +30,9 @@ bootstrap-update
 build <pkgname>
     Build package source (fetch + extract + configure + build).
 
+consistency-check
+    Runs a consistency check on all packages
+
 chroot
     Enter to the chroot in <masterdir>.
 
@@ -677,6 +680,9 @@ case "$XBPS_TARGET" in
             remove_pkg $XBPS_CROSS_BUILD
         fi
         ;;
+    consistency-check)
+        consistency_check
+        ;;
     remove-autodeps)
         if [ -n "$CHROOT_READY" -a -z "$IN_CHROOT" ]; then
             chroot_handler remove-autodeps