play: Print hex representation of key sequence in help.

play: Print hex representation of key sequence in help.

Some predefined keys of para_play, for example the four cursor keys,
are mapped to key sequences which should not be printed verbatim to
the console in com_help().

This patch introduces get_key_map_seq_safe(), an alternative to
get_key_map_seq() which returns the hexadecimal representation of the
bytes in the sequence of the given key. Single character sequences,
however, are printed verbatim if the character is printable. com_help()
is changed to call get_key_map_seq_safe() instead of get_key_map_seq().