adventure(6): sync to NetBSD and OpenBSD; cleanup

adventure(6): sync to NetBSD and OpenBSD; cleanup

- don't hide: no privileges -> no need to drop them
- sync manpage to OpenBSD
- split function declarations to extern.h
- sprinkle static and const (mostly from NetBSD)
- spell directions more consistently in glorkz (NetBSD)
- fix some off-by-ones (OpenBSD)
- flushline only if necessary
- use err(3) on out-of-memory, use calloc(3)
- improve list handling (via NetBSD from Coverity Prevent)
- return from main() (instead of calling exit)
- fix save file generation (OpenBSD)
- sort includes