mountd: Support junction management plug-ins

mountd: Support junction management plug-ins

To support FedFS and NFS junctions without introducing additional
build-time or run-time dependencies on nfs-utils, the community has
chosen to use a dynamically loadable library to handle junction
resolution.

There is one plug-in library for mountd that will handle any NFS-
related junction type.  Currently there are two types:

  o nfs-basic	locally stored file set location data, and

  o nfs-fedfs	file set location data stored on an LDAP server

mountd's support for this library is enabled at build time by the
presence of the junction API definition header:

  /usr/include/nfs-plugin.h

If this header is not found on the build system, mountd will build
without junction support, and will operate as before.

Note that mountd does not cache junction resolution results.  NFSD
already caches these results in its exports cache.  Thus each time
NFSD calls up to mountd, it is, in essence, requesting a fresh
junction resolution operation, not a cached response.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Steve Dickson <steved@redhat.com>