godbg: add ability to attach to an already running program

[sbinet]

leverage new gdblib.NewGDBWithPID feature and add -p PID flag.

• apply gofmt fixes.

[sirnewton01]

Thank you for the pull request. I will have a look at it.

One concern I had with the ability to attach to an existing Go process was how gdb behaves when the binary is not compiled with the debugging flags (see http://golang.org/doc/gdb). I have seen gdb hang, produce very strange information about variables, etc. when the binary wasn't compiled with the debug flags. This is why godbg currently attempts to recompile your binary if you provide it with the fully-qualified package name and not just the path to the binary.