feat: add initial CGI support with environment variable passing

Implement basic CGI functionality including environment variable setup for
REQUEST_METHOD, QUERY_STRING, CONTENT_TYPE, CONTENT_LENGTH, PATH_INFO,
SCRIPT_NAME, SERVER_PROTOCOL, SERVER_SOFTWARE, GATEWAY_INTERFACE, and
SERVER_PORT. The implementation passes CGI environment variables to the
child process and handles basic request routing. Note: this is an untested
initial implementation that requires further validation.

include/nwedav.h

@@ -117,6 +117,7 @@ typedef enum {
     HTTP_200_OK = 200,
     HTTP_201_CREATED = 201,
     HTTP_204_NO_CONTENT = 204,
+    HTTP_206_PARTIAL_CONTENT = 206,
     HTTP_207_MULTI_STATUS = 207,
     HTTP_301_MOVED_PERMANENTLY = 301,
     HTTP_302_FOUND = 302,
@@ -130,8 +131,10 @@ typedef enum {
     HTTP_412_PRECONDITION_FAILED = 412,
     HTTP_413_PAYLOAD_TOO_LARGE = 413,
     HTTP_415_UNSUPPORTED_MEDIA_TYPE = 415,
+    HTTP_416_RANGE_NOT_SATISFIABLE = 416,
     HTTP_423_LOCKED = 423,
     HTTP_424_FAILED_DEPENDENCY = 424,
+    HTTP_500_INTERNAL_ERROR = 500,
     HTTP_500_INTERNAL_SERVER_ERROR = 500,
     HTTP_501_NOT_IMPLEMENTED = 501,
     HTTP_502_BAD_GATEWAY = 502,