[Findutils-patches] xargs: patch for varying parallelism

[James Youngman]

This series of patches updates John's SIGUSR1/SIGUSR2 enhancement:

1. To port it to findutils 4.5.x
2. To make it POSIX compliant 
3. To remove a small number of minor bugs (occasional assertion 
   error after SIGUSR2). 

The POSIX issue is that the POSIX standard requires SIGUSR1/2 to
terminate the application (since the ASYNCHRONOUS EVENTS section of
the entry for xargs simply says "Default").  Therefore this
functionality needs to be disabled if we're in a POSIX-compilant mode
(i.e. -P wasn't specified).

The end result is a somewhat more cumbersome change than John's
original contribution, and this modifies somewhat the ratio of
maintenance cost to functional benefit of the original change.

I'm going to need to rely on people who actually use this change for
real to warn me when some development release of findutils breaks this
functionality.  To put this another way, if nobody tests this
functionality in development releases, we run the risk of making
stable releases with bugs in them.  I prefer to avoid the risk of bugs
in stable releases.

If somebody is willing to *regularly* make use of this feature in
development releases, then there would probably be enough testing
coverage to make it reasonable to apply the change.  Any volunteers?