opr: Add new softsig implementation

opr: Add new softsig implementation

Signals and pthreaded applications are a poor match. OpenAFS has had
the softsig system (currently in src/util/softsig.c) in an attempt to
alleviate some of these problems. However, that implementation itself
has a number of problems. It uses signal functions that are unsafe in
pthreaded applications, and uses pthread_kill within its signal
handlers. Over the years it has been responsible for a number of
portability bugs.

The old implementation continues to receive signals in the main thread
of the application. However, the handler code is run within a seperate
signal handler thread. When the main thread receives a signal a stub
handler is invoked, which simply pthread_kill()s the signal handler
thread.

The new implementation simplifies things by only receiving signals in
the handler thread. It uses only pthread-compatible signal functions,
and invokes no code from within async signal handlers.

A complete test suite is supplied.

Change-Id: I4bac68c2f853f1e7578b54ddced3833a97dd3f82
Reviewed-on: http://gerrit.openafs.org/6947
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Chas Williams <3chas3@gmail.com>
Reviewed-by: Daria Brashear <shadow@your-file-system.com>