Add unixsocket support for webinterface

[hkcomori]

Add unixsocket support for webinterface.
In environments where TCP port cannot be used for security reasons, Unix Socket support is required.

Note that if HTTP support is disabled, the Monit CLI interface will have reduced functionality, as most CLI commands (such as "monit status") needs to communicate with the Monit background process via the HTTP interface. We strongly recommend having HTTP support enabled. If security is a concern, bind the HTTP interface to local host only or use Unix Socket so Monit is not accessible from the outside.