FUSE drive permissions are fixed to user read/write only.

Member variable for selecting permissions level was added to drive so that
a mount option could be provided for selecting different permissions. CBFS
permissions to come later.
Removing UnixDriveTest.BEH_ToMode

The function ToMode in unix_drive.h currently invokes
fuse_get_context(). This function uses thread-local-storage which is not
being setup in the test case. The test is being removed until this
is resolved.
Replaced constexpr with MAIDSAFE_CONSTEXPR. Added to metadata tests.
Merge pull request #31 from vtnerd/MAID-270_FUSEUserPermissions

FUSE drive permissions are fixed to user read/write only.