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.c

@@ -199,9 +199,16 @@ gchar*
 mu_util_guess_mu_homedir (void)
 {
 	const char* home;
+	const gchar *hdir1;
 
-	/* g_get_home_dir use /etc/passwd, not $HOME; this is better,
-	 * as HOME may be wrong when using 'sudo' etc.*/
+	/* first, try MU_HOME */
+	hdir1 = g_getenv ("MU_HOME");
+
+	if (hdir1 && mu_util_check_dir (hdir1, TRUE, FALSE))
+		return g_strdup (hdir1);
+
+	/* then, g_get_home_dir use /etc/passwd, not $HOME; this is
+	 * better, as HOME may be wrong when using 'sudo' etc.*/
 	home = g_get_home_dir ();
 
 	if (!home) {