This is a meek client only implementation, with the following
differences with dcf's `meek-client`:

 - It is named `meek_lite` to differentiate it from the real thing.
 - It does not support using an external helper to normalize TLS
   signatures, so adversaries can look for someone using the Go
   TLS library to do HTTP.
 - It does the right thing with TOR_PT_PROXY, even when a helper is
   not present.

Most of the credit goes to dcf, who's code I librerally cribbed and
stole.  It is intended primarily as a "better than nothina" option
for enviornments that do not or can not presently use an external
Firefox helper.

Differences from my goptlib branch:
 * Instead of exposing a net.Listener, just expose a Handshake() routine
   that takes an existing net.Conn. (#14135 is irrelevant to this socks
   server.
 * There's an extra routine for sending back sensible errors on Dial
   failure instead of "General failure".
 * The code is slightly cleaner (IMO).

Gotchas:
 * If the goptlib pt.Args datatype or external interface changes,
   args.go will need to be updated.

Tested with obfs3 and obfs4, including IPv6.

Implements feature #15576.

If the relevant enviornment variable is set, treat read errors from
Stdin as a SIGTERM.

The ideal solution here would be to implement #15435, but till then
use one of several kludges:
 * Linux - prctl() so that the kernel SIGTERMs on parent exit.
 * Other U*ix - Poll the parent process id once a second, and SIGTERM
   ourself/exit if it changes.  Former is better since all the normal
   cleanup if any gets done.
 * Windows - Log a warning.