Enable Multi Socket Support

[avahahn]

Description

This commit refactors the CLI code to accept multiple instances of the control socket flag. All subcommands except for edit and save now support being run against multiple specified instances of unitd.

Changes

• control_socket_addresses CLI field is now a vector
• centralize error related logic into the error module
• wait_for_socket now returns a vector of sockets. all sockets in vector are waited upon and validated
• extraneous code is removed
• applications, execute, import, listeners, and status commands all run against N control sockets now
• edit and save commands return error when run against a single control socket

Fixes: #1318