Add a --daemon command-line parameter

When specifying --daemon, calcurse immediately forks and runs the
notification daemon in the background.

Signed-off-by: Lukas Fleischer <lfleischer@calcurse.org>

doc/manual.txt

@@ -200,6 +200,10 @@ second form is equivalent to `-Q --filter-type cal --days <num>`.
 Note: as for the `-a` flag, the calendar  from which to  read  the appointments
 can be specified using the `-c` flag.
 
+`--daemon`::
+  Start calcurse in background mode. See <<basics_daemon,Background mode>> for
+  details.
+
 `--days <num>`::
   Specify the length of the range (in days) when used with `-Q`. Cannot be
   combined with `--to`.