User defined alert messages

[Former user]

Add support for user's defined alert messages. Would be very helpful for user's script that return different exit codes.

[Tildeslash]

Scripts called from check program can already write an error message to stdout or stderr and it will be included in the alert message sent from Monit. Otherwise Monit also provides a way to compose the format of an alert message "freely" using $MONIT_XXX environment variables. The manual has more.