NFS: replace rpcinfo with showmounts

NFS: replace rpcinfo with showmounts

rpcinfo from rpcbind-0.2.1 in debian doesn't support ipv6 addresses.
At the same time the used command only actually tests for
portmapper/rpcbind availability, not for NFS directly.
Storage::scan_nfs uses /sbin/showmount to get a list of NFS exports from a
server and happily accepts ipv6 addresses. It is also more specific to NFS.

Replacing the rpcinfo call with showmount here means checking explicitly
for NFS and supporting IPv6 without the need for an updated rpcbind
package.