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.
NFS needs brackets around ipv6 addresses.
Also: nfs_is_mounted needs to quote the variables. This becomes apparent
when ipv6 addresses are used as then the address would otherwise be
treated as a character class, causing the check to always fail.
