gui: Improve config reload.

gui: Improve config reload.

There are two ways to make para_gui reload its config file: Sending
SIGUSR1 and running the internal reread_conf command. When SIGUSR1
is received, signal_post_select() calls com_reread_conf(). This is
a layer violation since command handlers should only be called from
the input task.

This patch provides the generic function reread_conf() which is called
from both the command handler and from signal_post_select().