fs: fat: create correct short names

The current function set_name() used to create short names has the
following deficiencies resolved by this patch:

* Long names (e.g. FOO.TXT) are stored even if a short name is enough.
* Short names with spaces are created, e.g. "A     ~1.TXT".
* Short names with illegal characters are created, e.g. "FOO++BAR".
* Debug output does not not consider that the short file name has no
  concluding '\0'.

The solution for the following bug is split of into a separate patch:

* Short file names must be unique.

This patch only provides the loop over possible short file names.

Fixes: c30a15e5 ("FAT: Add FAT write feature")
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>