psql: Improve portability of query cancel test

Some shells apparently don't support $PPID, so skip the test in that
case.

src/bin/psql/t/020_cancel.pl

@@ -25,14 +25,21 @@ SKIP: {
 
 	local %ENV = $node->_get_env();
 
+	my ($stdin, $stdout, $stderr);
+
+	# Test whether shell supports $PPID.  It's part of POSIX, but some
+	# pre-/non-POSIX shells don't support it (e.g., NetBSD, Solaris).
+	$stdin = "\\! echo \$PPID";
+	IPC::Run::run(['psql', '-X', '-v', 'ON_ERROR_STOP=1'], '<', \$stdin, '>', \$stdout, '2>', \$stderr);
+	$stdout =~ /^\d+$/ or skip "shell apparently does not support \$PPID", 2;
+
 	local $SIG{ALRM} = sub {
 		my $psql_pid = TestLib::slurp_file("$tempdir/psql.pid");
 		kill 'INT', $psql_pid;
 	};
 	alarm 1;
 
-	my $stdin = "\\! echo \$PPID >$tempdir/psql.pid\nselect pg_sleep(3);";
-	my ($stdout, $stderr);
+	$stdin = "\\! echo \$PPID >$tempdir/psql.pid\nselect pg_sleep(3);";
 	my $result = IPC::Run::run(['psql', '-X', '-v', 'ON_ERROR_STOP=1'], '<', \$stdin, '>', \$stdout, '2>', \$stderr);
 
 	ok(!$result, 'query failed as expected');