Attach a pty to each node's bash process

This should enable node commands that are expecting a tty to
behave better.
Avoid overhead of another process (env) in startShell()
Don't set self.waiting twice
Possibly faster check for sentinel.
Fix findPid since pty uses \r\n as line ending
Set default printPid back to True
Fix pid regex to eat \r\n
Restore non-mnexec pid detection for background commands
Merge branch 'master' into devel/pty
Reverting the disable signals change
Merge pull request #322 from mininet/devel/pty

Attach a pty to each node's bash process