- enable --detach, used by some .desktop files

Index: src/calibre/gui2/__init__.py
--- src/calibre/gui2/__init__.py.orig
+++ src/calibre/gui2/__init__.py
@@ -1119,7 +1119,7 @@ def load_builtin_fonts():
 
 
 def setup_gui_option_parser(parser):
-    if islinux:
+    if islinux or isbsd:
         parser.add_option('--detach', default=False, action='store_true',
                           help=_('Detach from the controlling terminal, if any (Linux only)'))
 
