Fix libpmi linking

libpmi was previously using the slurm_mutex_un/lock functions,
  which are dependent upon other slurm functions (e.g. "fatal()").
  Since this library is used by user applications and outside of
  slurm proper, we want to us the pthread_mutex_un/lock functions
  instead. Previous use of slurm functions was invoking glibc
  error() function rather than slurm's error() function and
  causing test7.2 to fail.