* errno.c: Use the GNU version of strerror_r if _GNU_SOURCE is defined

  * errno.c: Use the GNU version of strerror_r if _GNU_SOURCE is defined
    (otherwise assume existence of XPG variant).  This allows proper
    compilation under Red Hat 9.
  * fstab.c: protect against users calling setfsent(), setfsent(), ...
    endfsent(), which would otherwise leak a FILE handle.

svn path=/trunk/mono/; revision=42642