http: Don't try to destroy nonexistent sockets

Fixes #3740

In the case of pipelined requests, you can have a situation where
the socket gets destroyed via one req/res object, but then trying
to destroy *another* req/res on the same socket will cause it to
call undefined.destroy(), since it was already removed from that
message.

Add a guard to OutgoingMessage.destroy and IncomingMessage.destroy
to prevent this error.
install: Support $(PREFIX) install target directory prefix

This change introduces support for the common PREFIX variable in the
Makefile and install.py, instead of having /usr/local hardcoded. This
makes it much easier to install node to custom locations e.g. in a
user's home directory.

The PREFIX variable defaults to /usr/local.