stdenv/build-support: support .tbz and .txz tarballs

doc/stdenv.xml

···
         These can optionally be compressed using <command>gzip</command>
         (<filename>.tar.gz</filename>, <filename>.tgz</filename> or
         <filename>.tar.Z</filename>), <command>bzip2</command>
-        (<filename>.tar.bz2</filename> or <filename>.tbz2</filename>) or
-        <command>xz</command> (<filename>.tar.xz</filename> or
-        <filename>.tar.lzma</filename>).
+        (<filename>.tar.bz2</filename>, <filename>.tbz2</filename> or
+        <filename>.tbz</filename>) or <command>xz</command>
+        (<filename>.tar.xz</filename>, <filename>.tar.lzma</filename> or
+        <filename>.txz</filename>).
        </para>
       </listitem>
      </varlistentry>

pkgs/build-support/release/functions.sh

···
 findTarball() {
     local suffix i
     if [ -d "$1/tarballs/" ]; then
-        for suffix in tar.gz tgz tar.bz2 tbz2 tar.xz tar.lzma; do
+        for suffix in tar.gz tgz tar.bz2 tbz2 tbz tar.xz txz tar.lzma; do
             for i in $1/tarballs/*.$suffix; do echo $i; break; done
         done | sort | head -1
         return

pkgs/stdenv/generic/setup.sh

···
     else
 
         case "$fn" in
-            *.tar.xz | *.tar.lzma)
+            *.tar.xz | *.tar.lzma | *.txz)
                 # Don't rely on tar knowing about .xz.
                 xz -d < "$fn" | tar xf -
                 ;;
-            *.tar | *.tar.* | *.tgz | *.tbz2)
+            *.tar | *.tar.* | *.tgz | *.tbz2 | *.tbz)
                 # GNU tar can automatically select the decompression method
                 # (info "(tar) gzip").
                 tar xf "$fn"