Eliminate linear search for active parse tree

[rcmcdonald91]

This eliminates the need to perform a linear search every time cligen_pt_active_get is called. Instead we can just carry a pointer to the active pt_head structure on the cligen_handle and find it quickly when evaluating every line of input.

[olofhagsand]

Do you have some performance measurement, using just "time" or a perf analysis, to show a benefit of the optimization?

[rcmcdonald91]

good question, I will see about coming up with a test. This might indeed be a premature optimization, but it was something that caught my eye while studying the code.