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.

meson.build

@@ -116,6 +116,8 @@ endforeach
 
 if cc.has_function('wordexp')
   config_h_data.set('HAVE_WORDEXP_H',1)
+else
+  message('no wordexp, no command-line option expansion')
 endif
 
 testmaildir=join_paths(meson.current_source_dir(), 'lib', 'tests')