MEDIUM: http: add a small helper to compute how far to rewind to find headers

MEDIUM: http: add a small helper to compute how far to rewind to find headers

http_hdr_rewind() returns the number of bytes to rewind before buf->p to
find the beginning of headers. At the moment it's not exact as it still
relies on buf->o, assuming that no other data from a past message were
pending there, but it's what was done till there.

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