implement cload

fixing #43, extended by loading of multiple symbols, completion and the option to "query"; as all those is done using a cached worker module also added a "clean" for reloading.

Some TODOs are left in, the only thing I may add later here is a verify via readelf -s for the modules to check if those are actually COBOL modules (check for cob_module_enter), but I guess that's too much initial fileio so I left that out.

@rdubner I'd be happy if you could merge (and likely improve) that piece. @sjaak cload may be directly or indirectly useful for cbreak (maybe auto-try loading if the module is not found?)