lsfd: introduce XMODE column, extensible variant of MODE

lsfd: introduce XMODE column, extensible variant of MODE

The original MODE column comes from three letters (rwx) may not be
enough for representing various aspects of file descriptors and memory
mappings. We want to add more items in the future.

However, the description of MODE in lsfd(1) doesn't wrote about its
extensibility. Adding more letters to the column can break
compatibility in someone's use case.

This change introduces XMODE column. Unlike MODE, XMODE is declared as
an extensible column in lsfd(1). Currently, it shows the same items as
the MODE column does.

Signed-off-by: Masatake YAMATO <yamato@redhat.com>