efi_loader: cursor positioning

When backspacing in column 0 do no set the column index to ULONG_MAX.
Ensure that the row number is not set to ULONG_MAX even if the row count is
advertised as 0.
Ignore control characters other the 0x08, 0x0a, 0x0d when updating the
column.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>