exports: add a configurable time-to-live for the kernel cache entries

From: Trond Myklebust <Trond.Myklebust@netapp.com>

The fedfs ldap server will specify a ttl for its entries.

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>
This is a refactoring change only.  There should be no change in
behavior.

Original patch had updates to utils/mountd/junctions.c, which no
longer exists.  These are not included here.

Create a macro for the default cache TTL, which is used in several
places besides the export cache.

Make e_ttl unsigned.

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