Curses: fix extended command input

Curses: fix extended command input

The extended command input prompt was behaving in an unintended way:
Typing #a<enter> executed #adjust. Spaces in the entry prevented matching
any command. No error message was given when no command was matched.

Fix all of those, so it behaves more like the tty.

Clean up the tty, curses, and X11 windowport code, so they don't use
the extcmdlist array directly, but query with extcmds_match
and extcmds_getentry.