Warning cleanup: implicit function declarations in butc and bucoord

Warning cleanup: implicit function declarations in butc and bucoord

Cleanup some prototype related warnings under bucoord and butc:
- Copy a few prototypes from bucoord_internal.h to bucoord_prototypes.h,
since they are used in other source directories
- Install bc.h under include/afs, and include it in tcmain.c
- Include bucoord_prototypes.h in tcmain.c
- Add a prototype for GetResponseKey
- Remove warning inhibition for tcmain.c, and entry in README.WARNINGS
- Remove warning inhibition for bucoord/dump.c (no entry in README)

Change-Id: I0a0ae92d3c7ba75e4a331bf59ea9c031763cc704
Reviewed-on: http://gerrit.openafs.org/1134
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>