md_shellescape() sets SIGINT and SIGQUIT to be ignored, storing the
previous handlers, and restores them after the shell exits. But it
mixed up the two handlers.
Since the signals were usually handled by the same function, this fix
doesn't have much effect, but anything that makes signal code less
confusing is a good thing.
md_readchar() mapped KEY_BACKSPACE to CTRL-H, but get_str(), which
handles prompts for strings, only backs up when it receives the erase
character. The key should be mapped to md_erasechar().
This fixes Red Hat Bugzilla #847852.
