Fix the use of locks in the pause function

[rsanger]

The main problem is that we typically want to call pause(or stop which uses pause) from a signal handler, however in parallel libtrace this involves holding locks.

For a format like DPDK where trace_pstart takes some time and holds the lock the problem can easily be triggered.

Maybe it is best to block signals in pstart?

[dancollins]

I don't think blocking SIGTERM or SIGINT would be a good idea.
SIGINT it a useful way to end the program and it is cool when you output debug data at the end of a trace.
Maybe we use the signal to send a message? Failing that, maybe just set a global flag.