cfg_parse_ functions internally handle the parser

cfg_parse_ functions internally handle the parser

Instead of (1) allocating a parser, (2) parsing a file/buffer then (3)
freeing the parser, the parser is now internally created/destroyed from
within the `cfg_parse_*` functions. This simplifies a lot the use cases,
especially around the error cases where the parser needs to be freed in
a cleanup goto.

The only trick was the parser callback mechanism, which would previously
have been set up between steps 1 and 2.  Since it's never been used for
any purpose other than the "directory" option, the chdir call has now
been moved inside the parser and the generic callback mechanism has been
removed, replacing CFG_CLAUSEFLAG_CALLBACK with CFG_CLAUSEFLAG_CHDIR.