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