test/libcephfs: address windows issues

test/libcephfs: address windows issues

This commit addresses a few issues that prevent the libcephfs from running
on Windows:

* added missing definitions to fs_compat.h
* the "nobody" user is ignored on Windows, which is why we'll skip the tests
  that rely on it
* sys/mman.h and sys/resource.h are not available on Windows and will be skipped
* skip rlimit, not available on Windows
* fork is not available on Windows, for now we'll avoid compiling those tests
  when targetting Windows. For what is worth, those are skipped on other
  platforms anyway but currently fail at compile time when using mingw.
* use "strchr" instead of "index" or "strchrnul"

Signed-off-by: Lucian Petrut <lpetrut@cloudbasesolutions.com>