httpd: put all headers into environment as HTTP_UPPERCASED_HEADER=val

httpd: put all headers into environment as HTTP_UPPERCASED_HEADER=val

Set up environment variables before running the CGI script.
The variables will be named HTTP_<filtered_name> where the <filtered_name>
is the header name capitalized and all characters not matching
[a-z] | [A-Z] | [0-9] replaced with '_'.

function                                             old     new   delta
http_response                                         80      88      +8
http_response_type                                    20      22      +2
send_headers                                         718     715      -3
parse_conf                                          1481    1478      -3
get_line                                             128     110     -18
cgi_io_loop_and_exit                                 599     569     -30
send_cgi_and_exit                                    882     738    -144
handle_incoming_and_exit                            2793    2592    -201
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 2/6 up/down: 10/-399)          Total: -389 bytes
   text	   data	    bss	    dec	    hex	filename
 982178	    485	   7296	 989959	  f1b07	busybox_old
 981675	    485	   7296	 989456	  f1910	busybox_unstripped

Signed-off-by: Alexander Vickberg <wickbergster@gmail.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>