replace httpd_util functions removed in OTP26

See details here, the removed functions were replaced by proposed erlang functions which existed since very early releases and had the same behavior (I tested that

%%%    ... erlang:integer_to_list(X, 16) returns hex
%%%    values <10 as only 1 character, ie. "0F" is simply returned as
%%%    "F"* ...

), so the change must be ok with older OTP versions too.

On my Fedora 39 with OTP 26 sessions that contain apply_to_content='ts_digest:md5hex' break with

** Reason for termination ==
** {'function not exported',
       [{httpd_util,integer_to_hexlist,"Ô",[]},
        {ts_digest,'-tohex/1-fun-0-',1,
            [{file,"src/tsung/ts_digest.erl"},{line,75}]},
        {lists,map,2,[{file,"lists.erl"},{line,1559}]},
        {ts_digest,tohex,1,[{file,"src/tsung/ts_digest.erl"},{line,77}]},
        {ts_search,match,5,[{file,"src/tsung/ts_search.erl"},{line,142}]},
        {ts_client,handle_data_msg,2,
            [{file,"src/tsung/ts_client.erl"},{line,1150}]},
        {ts_client,handle_info2,3,
            [{file,"src/tsung/ts_client.erl"},{line,251}]},
        {gen_fsm,handle_msg,8,[{file,"gen_fsm.erl"},{line,475}]}]}

(in tsung installed by dnf), and worse, it hugely leaks memory. If I compile tsung from source, I get

$ make
Compiling  src/tsung/ts_digest.erl ... 
src/tsung/ts_digest.erl:75:44: Warning: httpd_util:integer_to_hexlist/1 is removed; use erlang:integer_to_list/2 with base 16 instead
%   75|                   ts_utils:to_lower(padhex(httpd_util:integer_to_hexlist(X)))
%     |                                            ^

This patch fixes the memory leak and the build.

processone / tsung

replace httpd_util functions removed in OTP26 #409