util.json: Accept empty arrays with whitespace (fixes #1782)

Merge 0.12->trunk

mod_http: Allow disabling CORS in the http_cors_override option and by default Fixes #1779. Due to an oversight in the logic, if the user set 'enabled' to false in an override, it would disable the item's requested CORS settings, but still apply Prosody's default CORS policy. This change ensures that 'enabled = false' will now disable CORS entirely for the requested item. Due to the new structure of the code, it was necessary to have a flag to say whether CORS is to be applied at all. Rather than hard-coding 'true' here, I chose to add a new option: 'http_default_cors_enabled'. This is a boolean that allows the operator to disable Prosody's default CORS policy entirely (the one that is used when a module or config does not override it). This makes it easier to disable CORS and then selectively enable it only on services you want it on.

Merge 0.12->trunk

mod_admin_shell: Ensure available connection for column 'secure' Similar to #1777

mod_admin_shell: Ensure connection exists to get port from (fixes #1777)

util.prosodyctl.cert: Remove Lua 5.1 os.execute() return value compat

util.prosodyctl: Remove Lua 5.1 os.execute() return value compat

util.openssl: Remove Lua 5.1 os.execute() return value compat

util.human.io: Fix handling of os.execute() return values in Lua 5.2+ Wrong part of Lua 5.1 compat removed in 0f4feaf9ca64