Allow conversion of characters in Mac remap range. Part 1

Allow conversion of characters in Mac remap range. Part 1

This allows directory listings to Mac to display filenames
correctly which have been created with illegal (to Windows)
characters in their filename. It does not allow
converting the other direction yet ie opening files with
these characters (followon patch).

There are seven reserved characters that need to be remapped when
mounting to Windows, Mac (or any server without Unix Extensions) which
are valid in POSIX but not in the other OS.

: \ < > ? * |

We used the normal UCS-2 remap range for this in order to convert this
to/from UTF8 as did Windows Services for Unix (basically add 0xF000 to
any of the 7 reserved characters), at least when the "mapchars" mount
option was specified.

Mac used a very slightly different "Services for Mac" remap range
0xF021 through 0xF027.  The attached patch allows cifs.ko (the kernel
client) to read directories on macs containing files with these
characters and display their names properly.  In theory this even
might be useful on mounts to Samba when the vfs_catia or new
"vfs_fruit" module is loaded.

Currently the 7 reserved characters look very strange in directory
listings from cifs.ko to Mac server.  This patch allows these file
name characters to be read (requires specifying mapchars on mount).

Two additional changes are needed:
1) Make it more automatic: a way of detecting enough info so that
we know to try to always remap these characters or not. Various
have suggested that the SFM approach be made the default when
the server does not support POSIX Unix extensions (cifs mounts
to Samba for example) so need to make SFM remapping the default
unless mapchars (SFU style mapping) specified on mount or no
mapping explicitly requested or no mapping needed (cifs mounts to Samba).

2) Adding a patch to map the characters the other direction
(ie UTF-8 to UCS-2 on open).  This patch does it for translating
readdir entries (ie UCS-2 to UTF-8)

Signed-off-by: Steve French <smfrench@gmail.com>
Reviewed-by: Shirish Pargaonkar <shirishpargaonkar@gmail.com>