Make target - proxy communication more secure.

Previously, the listening party just opened a socket and waited for any
TCP/IP connection. There was no authentication - any random process could
connect to the proxy or target, and start sending bogus commands. That has
two security problems:

1. It allows hijacking another user's debugging session.
2. The proxy and target are not very careful at checking the input coming
through the target <-> proxy connection, so you could send invalid input
and cause a server crash or a buffer overflow that might be exploitable
for privilege escalation.

A third problem is that the pldbg_attach_to_port(port) allowed connecting
to any local port in the system, whether or not it belonged to a debugging
session. That's not very serious, as you couldn't send arbitrary commands
through the connection, but better to plug that too.

It's still not a good idea to install pldebugger on a production server,
there's probably more vulnerabilities not covered by this, but this is
a good start at making it more robust.

To fix those issues, there's now a little shared memory structure that's
used to register connections belonging to pldebugger. Whenever a backend
initiates a connection or listens for a connection, it registers the port
number in shared memory. The other end can then check the shared memory
block to make sure it's connected to a legitimate debugging proxy or target.

This encapsulates the functions used to create connections between proxy
and target to new dbgcomm.c file. Similar to the breakpoint functions, the
functions in dbgcomm.c are used by both pldbgapi.so and plugin_debugger.so,
so plugin_debugger.so must be loaded before pldbgapi.so can be used.

Makefile

@@ -31,7 +31,7 @@ PLUGIN_big = plugin_debugger
 
 # OBJS lists the .o files comprising plugin_debugger.so. pldbgapi.so is
 # implicitly built from pldbgapi.c file.
-OBJS	   = plpgsql_debugger.o plugin_debugger.o
+OBJS	   = plpgsql_debugger.o plugin_debugger.o dbgcomm.o
 ifdef INCLUDE_PACKAGE_SUPPORT
 OBJS += spl_debugger.o
 endif