meek-client-torbrowser does not use signals well

When testing meek-client-wrapper, I noticed two things:

• it does not respond to SIGINT or SIGKILL. also, the signal handling code is different from meek-client. perhaps we should move it to goptlib?
• it uses sigkill to kill its children, not giving them a chance to clean up. Yes, this is awkward on windows but we can at least do something nicer on posix systems.