mount.nfs: Eliminate compiler warning in utils/mount/mount.c

Clean up.

In file included from mount.c:41:
mount_config.h:35: warning: no previous prototype for mount_config_opts

Functions defined in include files are usually declared as "static
inline," eliminating the need for a forward declaration.

While I was there, I also fixed the macro that prevents including
mount_config.h multiple times, and fixed some white space damage.

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