ddb: just move cursor when the lexer backs up

Get rid of db_look_char because it's not compatible with db_get_line().
This fixes the following issue:

db> script lockinfo=show alllocks
db> run lockinfo
db:0:lockinfo> how alllocks
No such command; use "help" to list available commands

Reported by: markj
Reviewed by: markj
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D28725