Reset skinny vmexten and immeddial char on reload.

Make skinny reset vmexten and immeddial to '\0' on reload to ensure that
it is set to '\0' if the appropriate item is removed/commented in 
skinny.conf. Also small fix re immeddial char in skinny.conf and add
immedial setting to skinny show settings.

(closes issue ASTERISK-21037)
Reported by: snuffy
Tested by: snuffy, myself
Patches: 
    immed_dial_fix.diff uploaded by snuffy (license 5024)


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@380925 65c4cc65-6c06-0410-ace0-fbb531ad65f3