nanosecond timestamp support in stat and utime

nanosecond timestamp support in stat and utime

* libguile/posix.h:
* libguile/posix.c (scm_utime): Add optional nanosecond arguments. This
  is an incompatible change on the C level, but it's unlikely people are
  using this POSIX wrapper function, because they would just use the
  POSIX function directly. Hopefully, anyway.

* module/system/base/compile.scm (call-with-output-file/atomic):
  Propagate source timestamps to targets with nanosecond precision, if
  available. Fixes build on systems with ext4 filesystems.

* libguile/filesys.c (scm_stat2scm):
* module/ice-9/posix.scm (stat:atimensec, stat:mtimensec)
  (stat:ctimensec): Add three new elements to Scheme stat structures,
  for nanosecond-level timestamps.

* configure.ac: Add checks for utimensat, and for nanosecond fields in
  struct stat. We should switch to using Gnulib things for these,
  though.

* doc/ref/posix.texi (File System): Add documentation for utime's
  additional arguments, and nanosecond stat timestamp accessors.