Support for NFS shares with spaces in path.

Unescape share address string listed in nfs_shares_config so that
'x.y.z.w:/foo\040bar' would become 'x.y.z.w:/foo bar' to support
mounting shares with spaces in name.
Use stat command to get available and total space on device instead
of df since it's output is easier to parse.

Fixes: bug #1180984
Change-Id: I552aba91ca8db14130f854c739594a818186bbe2
Merge "Support for NFS shares with spaces in path."