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")`.
This is essentially a convenience wrapper around the `pending()`
function, similar to `skip_fragile()` but more general-purpose.
Also remove `pending_win32` function as it can be replaced by
`skip(iswin())`.
- Partially fixes#10386 except for the case where the alternate buffer
is the default, empty, first buffer created on startup. #vimlife
- TODO: port patches related to `can_unload_buffer`, maybe that fully
fixes#10386?
vim-patch:8.0.1732: crash when terminal API call deletes the buffer
