Rename shellEscape -> escapeShellArgAlways

This name is close to the Nixpkgs lib function `escapeShellArg`,
making it easier to find.

A friendlier function with the same behavior as lib could be added
later.

src/libutil/util.cc

@@ -171,7 +171,7 @@ std::string toLower(std::string s)
 }
 
 
-std::string shellEscape(const std::string_view s)
+std::string escapeShellArgAlways(const std::string_view s)
 {
     std::string r;
     r.reserve(s.size() + 2);