stdenv: introduce baseHash() to replace stripHash()

stripHash uses a global variable to communicate it's computation
results, but it's not necessary. You can just pipe to stdout in a
subshell. A function mostly behaves like just another command.

baseHash() also introduces a suffix-stripping capability since it's
something the users of the function tend to use.

pkgs/data/sgml+xml/schemas/xml-dtd/docbook-ebnf/builder.sh

@@ -3,5 +3,4 @@ source $stdenv/setup
 mkdir -p $out/xml/dtd/docbook-ebnf
 cd $out/xml/dtd/docbook-ebnf
 cp -p $dtd dbebnf.dtd
-stripHash $catalog
-cp -p $catalog $strippedName
+cp -p $catalog $(baseHash $catalog)