Look in more places for the dovecot binary

fix #510

modoboa/extensions/admin/app_settings.py

@@ -3,6 +3,7 @@ from django.utils.translation import ugettext_lazy
 from modoboa.lib.formutils import YesNoField, SeparatorField
 from modoboa.lib.sysutils import exec_cmd
 from modoboa.lib import parameters
+import os
 
 
 class AdminParametersForm(parameters.AdminParametersForm):
@@ -47,8 +48,14 @@ class AdminParametersForm(parameters.AdminParametersForm):
         super(AdminParametersForm, self).__init__(*args, **kwargs)
         hide_fields = False
         code, output = exec_cmd("which dovecot")
+        known_paths = ("/usr/sbin/dovecot", "/usr/local/sbin/dovecot")
         if not code:
             dpath = output.strip()
+        else:
+            for fpath in known_paths:
+                if os.path.isfile(fpath) and os.access(fpath, os.X_OK):
+                    dpath = fpath
+        if dpath:
             try:
                 code, version = exec_cmd("%s --version" % dpath)
             except OSError: