Support NFS defaults on Solaris.

If `options` parameter is passed and it is set to 'defaults' on osfamily
Solaris, it will use '-' as the mount option instead of 'defaults', as
'defaults' is not supported on Solaris.
Merge pull request #5 from ghoneycutt/support_nfs_defaults_on_solaris

Support NFS defaults on Solaris.