net: rename internal functions for readability

The current namings are a bit confusing and hard to be searched for.

• Rename listen to listenInCluster
• Rename _listen2 to _setUpListenHandle
• Remove _listen since it's a one-liner only used in one place
• Correct comments in server.listen

Also Socket.prototype.listen doesn't seem to be tested nor documented, added a FIXME.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• commit message follows commit guidelines

Affected core subsystem(s)

net