add "rndc fetchlimit" to show fetchlimited servers

add "rndc fetchlimit" to show fetchlimited servers

this command runs dns_adb_dumpquota() to display all servers
in the ADB that are being actively fetchlimited by the
fetches-per-server controls (i.e, servers with a nonzero average
timeout ratio or with the quota having been reduced from the
default value).

the "fetchlimit" system test has been updated to use the
new command to check quota values instead of "rndc dumpdb".