lib,src,doc: add --heapsnapshot-signal CLI flag

[cjihrig]

This flag allows heap snapshots to be captured without modifying application code. IMO, this is a big part of the "heapdump in core" use case.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines

[nodejs-github-bot]

Lite-CI: https://ci.nodejs.org/job/node-test-pull-request-lite-pipeline/3196

[jasnell]

LGTM with @joyeecheung's comment addressed

[richardlau]

Does this work on Windows? (I'm guessing not, at least based on the equivalent test in

node/test/report/test-report-config.js

Lines 63 to 87 in cc4e8e0

	if(!common.isWindows){
	// Verify that process.report.signal behaves properly.
	assert.strictEqual(process.report.signal,'SIGUSR2');
	common.expectsError(()=>{
	process.report.signal={};
	},{code: 'ERR_INVALID_ARG_TYPE'});
	common.expectsError(()=>{
	process.report.signal='foo';
	},{code: 'ERR_UNKNOWN_SIGNAL'});
	assert.strictEqual(process.report.signal,'SIGUSR2');
	process.report.signal='SIGUSR1';
	assert.strictEqual(process.report.signal,'SIGUSR1');
	// Verify that the interaction between reportOnSignal and signal is correct.
	process.report.signal='SIGUSR2';
	process.report.reportOnSignal=false;
	assert.strictEqual(process.listenerCount('SIGUSR2'),0);
	process.report.reportOnSignal=true;
	assert.strictEqual(process.listenerCount('SIGUSR2'),1);
	process.report.signal='SIGUSR1';
	assert.strictEqual(process.listenerCount('SIGUSR2'),0);
	assert.strictEqual(process.listenerCount('SIGUSR1'),1);
	process.report.reportOnSignal=false;
	assert.strictEqual(process.listenerCount('SIGUSR1'),0);
	}

).

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/22362/

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/22383/

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/22395/

EDIT(cjihrig): CI was green.