In an upcoming commit, ovs-brcompatd will need to create a subprocess and
capture both its stdout and stderr separately, which one cannot do with
simple interfaces such as popen(). This function provides that ability.
To prevent fatal signals in a child process from causing the parent
process's pidfile, etc. to be deleted, we need to block fatal signals
around fork and call fatal_signal_fork() in the child process.
This problem was noticed through code inspection; it has not been observed
in practice.
