Reset dig exit code after a TCP connection is established

Reset dig exit code after a TCP connection is established

The "exitcode" variable is set to 9 if a TCP connection fails, but is
not reset to 0 if a subsequent TCP connection succeeds.  This causes dig
to return a non-zero exit code if it succeeds in getting a TCP response
after a retry.  Fix by resetting "exitcode" to 0 if connect_done()
receives an event with the "result" field set to ISC_R_SUCCESS.

(cherry picked from commit deb3b85cb2faba8fd72172a8df0e0631033196aa)