Improve mapping of attributes to UNIX permissions

[piranna]

Original issue with more in-detail explains at dorimanx/exfat-nofuse#118

Use FAT attributes and unused reserved bit fields to provide POSIX like permissions. Obviously permissions can't be the same, but workarounds can be find:

• ATTR_DEVICE field can be reused for execution permission, similar to what's being done by reusing the ATTR_VOLUME attribute field to add support for symlinks. This is the feature I'm more interested :-)
• ATTR_HIDDEN to disable read permission, similar to ATTR_READONLY being currently used by fatfs filesystem for read permission

Additionally, it could be possible to add multi-user in two ways:

• ATTR_SYSTEM to allow access only to the file only to root (or other "owner user") instead of everybody. It's simple to implement since It's like a simple users management system of priviledged user vs. anybody else
• use reserved fields to store UID and GID data, similar to how DR-DOS added users and access permissions. Problem here is that these exact bits are being used by ExFAT, but there are other free ones at the end of the structure