xbps-src: introduce a new target: "sort-dependencies".

This expects a variable argument list with package names that will be
returned to stdout topologically sorted.

$ ./xbps-src sort-dependencies libarchive-devel liblzma-devel libxbps zlib-devel bzip2-devel
bzip2
xz
zlib
libarchive
xbps

xbps-src

@@ -103,6 +103,10 @@ show-repo-updates
 show-sys-updates
     Prints the list of oudated packages in your system.
 
+sort-dependencies <pkg> <pkgN+1> ...
+    Given a list of packages specified as additional arguments, a sorted dependency
+    list will be returned to stdout.
+
 update-bulk
     Rebuilds all packages in the system repositories that are outdated.
 
@@ -381,7 +385,7 @@ while getopts "a:CEfgGhH:Ij:Lm:No:r:tV" opt; do
 done
 shift $(($OPTIND - 1))
 
-[ $# -eq 0 -o $# -gt 3 ] && usage && exit 1
+[ $# -eq 0 ] && usage && exit 1
 
 umask 022
 
@@ -691,6 +695,9 @@ case "$XBPS_TARGET" in
     show-sys-updates)
         bulk_build -i
         ;;
+    sort-dependencies)
+        bulk_sortdeps ${@/$XBPS_TARGET/}
+        ;;
     update-bulk)
         bulk_update
         ;;