Reserve two bytes at the end of the DDB input line in db_readline() to

Reserve two bytes at the end of the DDB input line in db_readline() to
hold the newline and nul terminator.  Otherwise, there are cases where
garbage may end up in the command history due to a lack of a nul
terminator, or input may end up without room for a newline.

MFC after:	3 days
PR:		119079
Submitted by:	Michael Plass <mfp49_freebsd@plass-family.net>