KeePassHTTP hotfix, resolves #147

[phoerious]

This pull request resolves #147. Although KeePassHTTP is still a weak protocol and its main flaws remain, this implementation should be safe enough for releasing it as part of KeePassXC.

Description

KeePassXC strictly limits communication to the local loopback interface at 127.0.0.1. This address is hardcoded and cannot be changed. The plugin is now also disabled by default at runtime and needs to be explicitly enabled before KeePassXC listens to any requests from PassIFox or chromeIPass.

Additional changes in this PR:

• Warn user when trying to bind to a port below 1024 and fall back to default port
• Update heavily outdated README.md and add security notice about KeePassHTTP
• Only require libmicrohttpd build-time dependency when compiling with HTTP support.

How Has This Been Tested?

There are still no unit tests for the HTTP plugin, but all functionality was tested manually.
The patch was tested both on my Arch system as well as in an Ubuntu 16.04 virtual machine.

Types of changes

• ✅ Bug fix (non-breaking change which fixes an issue) [Security Fix]

Checklist:

• ✅ I have read the CONTRIBUTING document. [REQUIRED]
• ✅ My code follows the code style of this project. [REQUIRED]
• ✅ All new and existing tests passed. [REQUIRED]

[TheZ3ro]

Not tested but code looks fine.