remove r->zero_in_uri

*) make $request_method non-cacheable
*) make $arg_ variables non-cacheable
*) change processing variables accessed by SSI and perl module:
   *) the indexed variables are always flushed
   *) never show warning for not found variables

perl related fixes:

*) do not log error if $r->variable was not found
*) delete ngx_http_perl_cleanup_t unused since r909

perl exit fixes:

*) use global perl variable in perl_destruct()/perl_free()
   for non-mulitiplicity perl
*) add comment from r2716 commit message

various perl fixes:

*) fix ngx_http_send_special() for subrequests handled by perl
*) allow perl "sub{..."
*) restore environ, this fixes segfault on reconfiguration failure when
   perl module creates new environment
*) optimize error handling
*) use ngx_conf_set_str_array_slot() for perl_require
*) allow several perl_modules