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.

man/mu.1

@@ -152,7 +152,9 @@ including \fBmu\fR without any command.
 .TP
 \fB\-\-muhome\fR
 causes \fBmu\fR to use an alternative directory to
-store and read its database and logs. By default, \fI~/.mu\fR is used.
+store and read its database and logs. By default, \fBmu\fR uses
+whatever the \fBMU_HOME\fR environment variable is set to; if it is
+not set, \fI~/.mu\fR is used.
 
 .TP
 \fB\-d\fR, \fB\-\-debug\fR