local_fs_map for DAP

[ko1]

On DAP via TCP/IP connection, all the source code are detected
as remote files (without localfs:true in launch.json for VSCode).
In this case, DAP clients (VSCode, ...) receive the contents
of the file from DAP server and open an editor window in readonly mode.

local_fs_map configuration can set remote->local path translation.

For example,

RUBY_DEBUG_LOCAL_FS_MAP=/remote/src/:/local/src/ rdbg -O --port=...

On this case, if remote path is /remote/src/is/foo.rb, then DAP
returns /local/src/is/foo.rb and open the local file as writable
mode.

You can specify multiple path maps with ',' character like

RUBY_DEBUG_LOCAL_FS_MAP=/r1/:/l1/,/r2/:/l2/ ...

and also you can specify this map within launch.json for VSCode like:

        {
            "type": "rdbg",
            "name": "Attach with rdbg (TCP/IP 12345)",
            "request": "attach",
            "debugPort": "localhost:12345",
            "localfsMap": "/remote/src/:/local/src/"
        }

If both are provided, launch.json setting is used.

fix #463
fix ruby/vscode-rdbg#32