process: block signals while spawning child processes

Between fork() and execvp() calls in the process_start()
function both child and parent processes share the same
file descriptors.  This means that, if a child process
received a signal during this time interval, then it could
potentially write data to a shared file descriptor.

One such example is fatal signal handler, where, if
child process received SIGTERM signal, then it would
write data into pipe.  Then a read event would occur
on the other end of the pipe where parent process is
listening and this would make parent process to incorrectly
believe that it was the one who received SIGTERM.
Also, since parent process never reads data from this
pipe, then this bug would make parent process to consume
100% CPU by immediately waking up from the event loop.

This patch will help to avoid this problem by blocking
signals until child closes all its file descriptors.

Signed-off-by: Ansis Atteka <aatteka@nicira.com>
Reported-by: Suganya Ramachandran <suganyar@vmware.com>
Issue: 1255110