Allow setting allowmultiplelogin on an account basis

This patch modifies manager to allow the allowmultiplelogin setting to be set
on an account by account basis. When set in the general context, it will act
as the default for the defined accounts. Setting it in the account will
override the general setting.

(closes issue ASTERISK-21324)
Reported by: vldmr
patches:
  asterisk-manager-per-user-allowmultiplelogin.patch uploaded by vldmr (License 6487)

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@394881 65c4cc65-6c06-0410-ace0-fbb531ad65f3