MINOR: h3: refactor SETTINGS parsing/error reporting

MINOR: h3: refactor SETTINGS parsing/error reporting

Bring some improvment to h3_parse_settings_frm() function. The first one
is the parsing which now manipulates a buffer instead of a plain char*.
This is more to unify with other parsing functions rather than dealing
with data wrapping : it's unlikely to happen as SETTINGS is only
received as the first frame on the control STREAM.

Various errors are now properly reported as connection error :
* on incomplete frame payload
* on a duplicated settings in the same frame
* on reserved settings receive