Config: Make the parser and lexer reentrant.

This is part of the multithreading journey. The parser and lexer were
using loads of global variables and all of these are now packed into
struct cf_context and others.

Note that the config API has changed:

* cfg_alloc[zu]?(size) is now cf_alloc[zu]?(ctx, size)
* cf_error(msg, ...) is now cf_error(ctx, msg, ...)
* config_parse() and cli_parse() are now called differently
* there is a brand new CF_CTX section in *.Y files which participates
  in struct cf_context construction