Add fixes and cleanup to app_alarmreceiver.

Add fixes and cleanup to app_alarmreceiver.

This work comes courtesy of Pedro Kiefer (License #6407)
The work was posted to review board by Kaloyan Kovachev (License #5506)

(closes issue ASTERISK-16668)
Reported by Grant Crawshay

(closes issue ASTERISK-16694)
Reported by Fred van Lieshout

(closes issue ASTERISK-18417)
Reported by Kostas Liakakis

(closes issue ASTERISK-19435)
Reported by Deon George

(closes issue ASTERISK-20157)
Reported by Pedro Kiefer

(closes issue ASTERISK-20158)
Reported by Pedro Kiefer

(closes issue ASTERISK-20224)
Reported by Pedro Kiefer

Review: https://reviewboard.asterisk.org/r/2075

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@372310 65c4cc65-6c06-0410-ace0-fbb531ad65f3