func_evalexten: Add EVAL_SUB function.

func_evalexten: Add EVAL_SUB function.

This adds an EVAL_SUB function, which is similar to the existing
EVAL_EXTEN function but significantly more powerful, as it allows
executing arbitrary dialplan and capturing its return value as
the function's output. While EVAL_EXTEN should be preferred if it
is possible to use it, EVAL_SUB can be used in a wider variety
of cases and allows arbitrary computation to be performed in
a dialplan function call, leveraging the dialplan.

Resolves: #951