conf.c: extract ini parsing code to separate files

Move _parseconfig to ini.c as _parse_ini and create a convenient wrapper
for the public API.

Signed-off-by: Andrew Gregory <andrew.gregory.8@gmail.com>
ini.c: move recursion limit to a macro

Signed-off-by: Andrew Gregory <andrew.gregory.8@gmail.com>
ini.c: reuse line buffer

By the time we make the recursive call we have already finished with the
line buffer, making it safe to reuse.

Signed-off-by: Andrew Gregory <andrew.gregory.8@gmail.com>
ini.c: make errors in includes fatal

If an error in the main file would be fatal there is little reason to
ignore the error in an included file.

Signed-off-by: Andrew Gregory <andrew.gregory.8@gmail.com>