Add option to try to connect to a range of TCP ports

[gerymate]

Add option to try to connect to a range of TCP ports

Fixes: #368 Fixes: #1117

Description

We can set an integer value with --port-range or RUBY_DEBUG_PORT_RANGE in addition to setting a TCP port for remote debugging. When port is unavailable, rdbg will try to open port + 1, port + 2, etc. until it finds a suitable port, or reaches the length of the range. This makes it possible to remote debug multiple debuggees in a multithreaded environment, but still use TCP ports in a determined way.