Handling fatal signals in GNU make

There are a number of Savannah bugs around GNU make's handling of fatal
signals.  There can be deadlocks, etc.  The fundamental problem is that
when make receives a fatal signal it will run the entire shutdown from
within the signal handler.  That can lead to a lot of problems.

The obvious solution is to not do this and instead just set a flag or
similar in the signal handler, then check this flag in the main code.

Of course this is not so simple: as we well know from previous
wrestling with this it's very difficult to handle both waiting and
signal handling in a sane way.  We managed this in more modern systems
by using pselect() and in older systems with a trick of closing the
jobserver pipe.

On systems that support pselect() we could try to use it to create
critical sections for fatal signals in addition to SIGCHLD: block fatal
signals, check the "received fatal signal" flag, then use pselect() to
re-enable while waiting.  Of course we still need to introduce checks
of the "fatal signal received" flag around the code so we can react to
it even when we're doing things other than waiting for children (or
jobserver tokens).  And, we would need to increase the usage of
pselect() to more places that we might wait; currently it's only used
when we want to wait for jobserver tokens.

Another thought I had: we have already wrapped most system calls with
the EINTRLOOP macro.  One possibility would be to extend this macro so
it does the check of the "received fatal signal" flag, then disable
SA_RESTART and rely on this loop in all situations.  We might need to
review the code to make sure we haven't missed places that need it. 
This is seriously old-school: I guess this is how signals had to be
managed originally?

And, I haven't given too much thought so far to Windows support; I'm
not sure how compatible it is with this model of signal handling.

An even more adventurous option would be to change make to be
multithreaded.  To really solve this problem using multithreading would
require a significant structural change, though.