msg_puts_display was more complex than necessary in nvim, as in
nvim, it no longer talks directly with a terminal.

In particular we don't need to scroll the grid before emiting the last
char. The TUI already takes care of things like that, for terminals
where it matters.

Co-authored-by: zeertzjq <zeertzjq@outlook.com>
Co-authored-by: famiu <famiuhaque@protonmail.com>

This avoids running title tests twice unnecessarily.

Problem:
When not inside an Ex command, screen_resize() calls update_screen(),
which calls screenclear() and set the screen as valid. However, when
inside an Ex command, redrawing is postponed so update_screen() screen
doesn't do anything, and the screen is still invalid after the resize,
causing ui_comp_raw_line() to be no-op until update_screen() is called
on the main loop.

Solution:
Restore the call to screenclear() inside screen_resize() so that the
screen is invalid after screen_resize(). Since screenclear() changes
redraw type from UPD_CLEAR to UPD_NOT_VALID, it is called at most once
for each resize, so this shouldn't change how much code is run in the
common (not inside an Ex command) case.

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")`.