Integration of EXEC preprocessor beyond baseline.

[utam-1]

Description of PR

This is a follow - up to #279.

Approach and basic idea

cb_preparser_entry is added to cobc.c, which handles the associated pre-parser metadata. cb_preparser_list and cb_active_preparser are created, one being a list to registered preparsers, and other is a pointer for pplex.l to identify the tag.

cb_load_preparser_conf() and cb_find_preparser() are created based on similar mechanism of cb_config_entry().
In ppparse.y the following macro is defined #define pperror(x) do { if (!cb_active_preparser) cb_error_always ("%s", x); } while(0) the idea is to identify the abort of pre-parser in pplex.l which is artificially generated.

So far only general tests have been performed by using a test.conf file and setting up a pseudo - preprocessor via bash, which shows the replacement works and flag-injection is performed via gcc.