MINOR: http: add two header parsing functions

MINOR: http: add two header parsing functions

Add http_next_hdr_value() to iterate over the comma-separated values
that some headers contain. Add http_get_hdr_param() to iterate over the
';'-separated parameters that may follow a value.

Currently unused but will be used for parsing Link headers in the cache
to support 103 Early Hints, and could also clean up Cache-Control
parsing among others.