REORG: http: move some very http1-specific parts to h1.{c,h}

REORG: http: move some very http1-specific parts to h1.{c,h}

Certain types and enums are very specific to the HTTP/1 parser, and we'll
need to share them with the HTTP/2 to HTTP/1 translation code. Let's move
them to h1.c/h1.h. Those with very few occurrences or only used locally
were renamed to explicitly mention the relevant HTTP version :

  enum ht_state      -> h1_state.
  http_msg_state_str -> h1_msg_state_str
  HTTP_FLG_*         -> H1_FLG_*
  http_char_classes  -> h1_char_classes

Others like HTTP_IS_*, HTTP_MSG_* are left to be done later.