MINOR: Makefile: provide cscope rule

"make cscope" builds tags for cscope.
CLEANUP: The function "regex_exec" needs the string length but in many case they expect null terminated char.

If haproxy is compiled with the USE_PCRE_JIT option, the length of the
string is used. If it is compiled without this option the function doesn't
use the length and expects a null terminated string.

The prototype of the function is ambiguous, and depends on the
compilation option. The developer can think that the length is always
used, and many bugs can be created.

This patch makes sure that the length is used. The regex_exec function
adds the final '\0' if it is needed.
MINOR: http: some exported functions were not in the header file

Export the following functions:
 - find_hdr_value_end
 - http_header_match2
 - http_remove_header2
 - http_header_add_tail2
MINOR: http: change url_decode to return the size of the decoded string.

Currently url_decode returns 1 or 0 depending on whether it could decode
the string or not. For some future use cases, it will be needed to get the
decoded string length after a successful decoding, so let's make it return
that value, and fall back to a negative one in case of error.
BUILD/MINOR: missing header file

In the header file "types/proto_http.h", the list are used
but the header file "mini-clist.h" is not included.