Decouple all global variables and structures

[Theldus]

Description

The current state of libag uses dozens of variables, arrays, and global structures, which just reflect the original ag code.

The problem is that this prevents several things, such as:

• Multiple search settings at the same time
• Simultaneous searches (issue Parallel thread-safe implementation of ag_search() #1)

This is the biggest dependency and hurdle to having a good ag(1) port to library.

However, I don't expect this to be resolved in a single PR, as it covers too many things on the same subject. I, therefore, hope that problems like the above are resolved first and so there will be little to work on afterward.