Read /etc/exports.d/*.export as extra export files

This patch adding a capability to read /etc/exports.d/*.exports as
extra export files to exportfs.

If one wants to add or remove an export entry in a script, currently
one may have to use sed or something tool for adding or removing the
line for the entry in /etc/exports file.

With the patch, adding and removing an entry from a script is much
easier.
cat<<EOF... or mv can be used for adding. rm can be used for removing.

Signed-off-by: Masatake YAMATO <yamato@redhat.com>
Signed-off-by: Steve Dickson <steved@redhat.com>