MFC discussed with: jilles, -developers

MFC discussed with: jilles, -developers

MFC r266971:

  - Return NULL and set errno to EINVAL if size is 0 (as required by POSIX).
  Update the manpage to reflect this change.
  - Always set the current position to the first null-byte when opening in append
  mode. This makes the implementation compatible with glibc's. Update the test
  suite.

  Reported by:	pho
  Approved by:	cognet

git-svn-id: svn://svn.freebsd.org/base/stable/10@277016 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f