MINOR: http: extract content-length parsing from H2

MINOR: http: extract content-length parsing from H2

Extract function h2_parse_cont_len_header() in the generic HTTP module.
This allows to reuse it for all HTTP/x parsers. The function is now
available as http_parse_cont_len_header().

Most notably, this will be reused in the next bugfix for the H3 parser.
This is necessary to check that content-length header match the length
of DATA frames.

Thus, it must be backported to 2.6.