merge r2972, r2994, r3133, r3142, r3143, r3174, r3175, r3176:

merge r2972, r2994, r3133, r3142, r3143, r3174, r3175, r3176:

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