update_condensed_str(): Fix for empty input strings

Empty input strings resulted in unterminated result strings.
Verify: Fix and improve verify_header()

Report errors directly from verify_header(); requires passing in the header
number and length.  Correctly report the file name, offset, and expected header
size in all error messages.
Add configure flags for gfapi and rbd

Add --disable-{gfapi,rbd} to disable gfapi/rbd support even if available.

Signed-off-by: Jens Axboe <axboe@fb.com>