gnu: mg: Upgrade to 20161005.

* gnu/packages/mg.scm (mg)[source]: Upgrade to 20161005.
[native-inputs]: Add pkg-config.
[inputs]: Add libbsd.
[arguments]: Use modify-phases.  Use #:make-flags rather than patching the
Makefile.  Install tutorial.
[description]:  Clarify relation to GNU Emacs.