mutt: update to 1.5.23

This commit rewrites the expression somewhat:

- it tries to simplify the expression.
- it changes from fetchurl to fetchhg.
- a patch to configure.ac is added in order to enable configuring the
  source on NixOS.
mutt: added mailpath.patch.
Merge pull request #3164 from 1126/master

mutt: update to 1.5.23