configure: cleanups for nfs ioengine

configure: cleanups for nfs ioengine

By default, build the nfs ioengine if configure can find libnfs.

Add a --disable-libnfs to avoid building the nfs ionegine even if
support could be found on the system.

Change the --enable-libnfs option to abort the build if libnfs cannot be
found.

Also use pkg-config to find the libnfs path instead of hard coding the
path.

Finally, drop CONFIG_NFS and use CONFIG_LIBNFS everywhere. There's no
reason to have two separate symbols.

Signed-off-by: Vincent Fu <vincentfu@gmail.com>