plugin_read_all: Check the `loop' variable after each iteration and return early...

plugin_read_all: Check the `loop' variable after each iteration and return early if the daemon is shutting down.

Lubos Stanek has reported that the daemon may stop too slow, resulting in it
being killed by impatient init-scripts. With this patch `plugin_read_all'
checks the `loop' variable before entering each plugin's read-function. If the
loop-variable is set to non-zero it will return early, allowing the daemon to
shutdown more instantly.