[Bug 6069] Add a fstatvfs function for libsmbclient

[Bug 6069] Add a fstatvfs function for libsmbclient

- Reverse the sense of the flags. Since the fstatvfs() function on POSIX-like
  systems would almost certainly indicate case sensitivity (for example),
  leave the bit turned off if the result is the POSIX-like result just as if
  issued on a typical local file system on a POSIX system.

Derrell
(cherry picked from commit 0acc962e3968253a3f64b5a92def177ced44994d)