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.

lib/mu-util.h

@@ -62,7 +62,8 @@ char* mu_util_guess_maildir (void)
 
 
 /**
- * guess the place of the mu homedir (typically, ~/.mu). Note, this
+ * guess the place of the mu homedir; first try $MU_HOME; if it is
+ * unset or non-existant, try ~/.mu. Note, the fallback ~/.mu
  * directory does not necessarily exist. mu_util_check_dir can be used
  * to check that
  *