Add --no-exit to keep running even after the subprocess exits

[alexflint]

A subprocess launched by httptap can "daemonize" itself (fork, detach itself with setsid, then fork again), in which case the subprocess launched by httptap will return, but other processes will still be running in the network namespace. At present, httptap exits when the immediate subprocess exits, leaving other such processes stranded in an isolated network namespace. Those processes will have no network connectivity, and the user won't see HTTP calls they make on the command line. This PR adds a simple workaround: a --no-exit flag that simple has httptap wait around until it is killed.

A good minimal way to reproduce this phenomenon is:

$ httptap -- setsid setsid curl http://httpbin.org/get
curl: (6) Could not resolve host: httpbin.org

A python example that illustrates a bit more what syscalls are at play is:

$ httptap -- python -c 'import os; import sys; import subprocess
if os.fork(): sys.exit()
os.setsid()
if os.fork(): sys.exit()
subprocess.run(["curl", "http://httpbin.org/get"])'
curl: (6) Could not resolve host: httpbin.org

An existing workaround is to put a "sleep infinity" into the command run by httptap:

$ httptap -- bash -c "setsid setsid curl http://httpbin.org/get; sleep infinity"
---> GET http://httpbin.org/get
<--- 200 http://httpbin.org/get (285 bytes)
...

The above works but is a bit obscure. With this PR this becomes:

$ httptap --no-exit -- setsid setsid curl http://httpbin.org/get                                                                      
---> GET http://httpbin.org/get
<--- 200 http://httpbin.org/get (285 bytes)

The user still has to press ctrl+c at the end.

#63