Add support for AIX systems:

* Memory leak in slurm_cred.c, added EVP_MD_CTX_cleanup().
* Pthread stack size too small on AIX. Resulting in stack corruption
  and ugly failure modes. Added slurm_attr_init to macros.h to
  explicitly set the stack size  for all pthreads.
* /dev/urandom not present on AIX, use rand() as needed instead in
  constructing a credential. Used in "srun --join".
* getsockopt(Socket, Level, SO_ERROR, &err, OptionLenght) sometime
  returns an error code of -1. This causes an assert failure in
  slurmd/io.c:_update_error_state().
* Function aliasing is not working on AIX. It is being turned off
  via a variable in config.h and "#if" logic in macros.h and
  slurm_xlator.h.
* dlopen failing if plugins reference any functions not present in
  caller. This may be fixed with the LDFLAG "-Wl,-bgcbypass=1000"
  being added for the slurm commands (avoid garbage collection of
  unused functions).
* read() is sometimes generates EAGAIN error, which was not handled
  in some places.
* vsnprintf() for string NULL is printing "" instead of "(null)"
  as produced by snprintf(). More format printing was added to log.c
  to produce more consistent log messages.
* poll() takes a timeout of -1 for unlimited rather than any negative
  number. Modify logic that was always multiplying by 1000 to convert
  usec to msec.
* getopt_long keyword table was not NULL terminated, resulting in
  segfault with invalid command-line argument in most commands.
* xmalloc module assert failures were not generating a core file.
  Changed "fatal();abort();" to "error();abort();".
* Change msg timeout from 3 sec to 5 sec. Running everything on single
  AIX node was very slow.