http*: move common variables and macros to http.[ch]

http*: move common variables and macros to http.[ch]

Move RANGE_HEADER_SIZE to http.h.

Create no_pragma_header, the curl header list containing the header
"Pragma:" in http.[ch]. It is allocated in http_init, and freed in
http_cleanup. This replaces the no_pragma_header in http-push.c, and
the no_pragma_header member in walker_data in http-walker.c.

Create http_is_verbose. It is to be used by methods in http.c, and is
modified at the entry points of http.c's users, namely http-push.c
(when parsing options) and http-walker.c (in get_http_walker).

Signed-off-by: Tay Ray Chuan <rctay89@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>