Problem:    Using freed memory when executing delfunc at the more prompt.
Solution:   Check function list not changed in another place. (closes vim/vim#11437)

https://github.com/vim/vim/commit/398a26f7fcd58fbc6e2329f892edbb7479a971bb

Co-authored-by: Bram Moolenaar <Bram@vim.org>

test: replace lfs with luv

luv already pretty much does everything lfs does, so this duplication
of dependencies isn't needed.

Extend the capabilities of is_os to detect more platforms such as
freebsd and openbsd. Also remove `iswin()` helper function as it can be
replaced by `is_os("win")`.

Problem:    Crash when trying to use s: variable in typed command.
Solution:   Don't use the script index when not set. (Ken Takata,
            closes vim/vim#6366)

https://github.com/vim/vim/commit/8e6cbb72324b6fb25d1a9abd6cc4d102d0e5f14e

`!did_throw` doesn't exactly imply `!current_exception`, as `did_throw = false`
is sometimes used to defer exception handling for later (without forgetting the
exception). E.g: uncaught exception handling in `do_cmdline()` may be deferred
to a different call (e.g: when `try_level > 0`).

In #7881, `current_exception = NULL` in `do_cmdline()` is used as an analogue of
`did_throw = false`, but also causes the pending exception to be lost, which
also leaks as `discard_exception()` wasn't used.

It may be possible to fix this by saving/restoring `current_exception`, but
handling all of `did_throw`'s edge cases seems messier. Maybe not worth
diverging over.

This fix also uncovers a `man_spec.lua` bug on Windows: exceptions are thrown
due to Windows missing `man`, but they're lost; skip these tests if `man` isn't
executable.