REORG: http: create http_msg.c to place there some legacy HTTP parts

Lots of HTTP code still uses struct http_msg. Not only this code is
still huge, but it's part of the legacy interface. Let's move most
of these functions to a separate file http_msg.c to make it more
visible which file relies on what. It's mostly symmetrical with
what is present in http_htx.c.

The function http_transform_header_str() which used to rely on two
function pointers to look up a header was simplified to rely on
two variants http_legacy_replace_{,full_}header(), making both
sides of the function much simpler.

No code was changed beyond these moves.
diff --git a/Makefile b/Makefile
index 141a6e6..587bc04 100644
--- a/Makefile
+++ b/Makefile
@@ -947,7 +947,7 @@
        src/http.o src/hpack-dec.o src/action.o src/proto_udp.o src/http_acl.o \
        src/xxhash.o src/hpack-enc.o src/h2.o src/freq_ctr.o src/lru.o         \
        src/protocol.o src/arg.o src/hpack-huff.o src/hdr_idx.o src/base64.o   \
-       src/hash.o src/mailers.o src/activity.o
+       src/hash.o src/mailers.o src/activity.o src/http_msg.o
 
 EBTREE_OBJS = $(EBTREE_DIR)/ebtree.o $(EBTREE_DIR)/eb32sctree.o \
               $(EBTREE_DIR)/eb32tree.o $(EBTREE_DIR)/eb64tree.o \