Add support for specifying --muhome via MU_HOME in the environment

When you have multiple mu home directories, e.g. for the use case
detailed in my "Changing mu4e-{maildir,mu-home} from a context hook"
post to the mailing list it's quite inconvenient to have to hammer out
"mu --muhome=.. find .." every time you want to run some ad-hoc
command.

This allows me to set up a screen session where I do searches in mu
directory A in some screen panes, and searches in directory B in
others.

I initially called this MU_MUHOME but then I noticed that the perl
plugin has MUP_MU_HOME for analogous functionality, so I'm just
following its example.

The code I'm adding in mu-util.c is just a copy/paste & adjustment of
the same sort of already tested functionality in
mu_util_guess_maildir() just a few lines earlier.

NEWS.org

@@ -16,6 +16,9 @@
    - prefer gpg2 for crypto
    - fix a crash when running on OpenBSD
    - fix --clear-links (broken filenames)
+   - You can now set the MU_HOME environment variable as an
+     alternative way of setting the mu homedir via the --muhome
+     commadn-line parameter.
     
 *** mu4e