MINOR: http: add a small helper to compute how far to rewind to find URI

MINOR: http: add a small helper to compute how far to rewind to find URI

http_uri_rewind() returns the number of bytes to rewind before buf->p to
find the URI. It relies on http_hdr_rewind() to find the beginning and
is just here to simplify operations.

The purpose is to centralize further ->sov changes aiming at avoiding
to rely on buf->o.