options: implement ExpandPath transformer

For expanding shell options (with expand_path / wordexp)

Note that e.g. in zsh: --maildir=~/Maildir is handled (program receives
--maildir=/home/user/Maildir) but e.g. bash does not do that, and the
program receives the literal '~/Maildir'

We expanded this in mu earlier, so let's do that again.

mu/mu-options.cc

@@ -43,6 +43,7 @@
 #include <unistd.h>
 
 #include <utils/mu-utils.hh>
+#include <utils/mu-utils-file.hh>
 #include <utils/mu-error.hh>
 #include "utils/mu-test-utils.hh"
 #include "mu-options.hh"
@@ -136,6 +137,19 @@ options_map(const IE& ie)
 	return map;
 }
 
+
+
+// transformers
+
+
+// Expand the path using wordexp
+static const std::function ExpandPath = [](std::string filepath)->std::string {
+	if (auto&& res{expand_path(filepath)}; !res)
+		throw CLI::ValidationError{res.error().what()};
+	else
+		return filepath = std::move(res.value());
+};
+
 /*
  * common
  */