Support for specifying a proxy for all outgoing connections

[Kovah]

Add support for specifying a proxy for all outgoing connections to bypass corporate firewalls.

Discussed in https://github.com/Kovah/LinkAce/discussions/452

[joshdvir]

You can use - https://www.cyberciti.biz/faq/linux-unix-set-proxy-environment-variable/ in Linux, nothing to implement in the application