Add bindings to GNU `sched_setaffinity' and `sched_getaffinity'.

Add bindings to GNU `sched_setaffinity' and `sched_getaffinity'.

* configure.ac: Add checks for `sched_setaffinity' and
  `sched_getaffinity'.

* doc/ref/posix.texi (Processes): Document `getaffinity' and
  `setaffinity'.

* libguile/posix.c (cpu_set_to_bitvector,
  scm_getaffinity)[HAVE_SCHED_GETAFFINITY]: New functions.
  (scm_setaffinity)[HAVE_SCHED_SETAFFINITY]: New function.

* libguile/posix.h (scm_getaffinity, scm_setaffinity): New declarations.

* test-suite/tests/posix.test ("affinity"): New test prefix.