Make psql's qeury canceling test simple by using signal() routine of IPC::Run

Hello,

Currently, the psql's test of query cancelling (src/bin/psql/t/020_cancel.pl)
gets the PPID of a running psql by using "\!" meta command, and sends SIGINT to
the process by using "kill". However, IPC::Run provides signal() routine that
sends a signal to a running process, so I think we can rewrite the test using
this routine to more simple fashion as attached patch.

What do you think about it?

Use of signal() is suggested by Fabien COELHO during the discussion of
query cancelling of pgbench [1].

[1] https://www.postgresql.org/message-id/7691ade8-78-dd4-e26-135ccfbf0e9%40mines-paristech.fr

Regards,
Yugo Nagata

--
Yugo NAGATA <nagata(at)sraoss(dot)co(dot)jp>