Problem:
:Man command errors if given more than two arguments. Thus, it is
impossible to open man pages that contain spaces in their names.

Solution:
Adjust :Man so that it tries variants with spaces and underscores, and
uses the first found.

fix(man): use italics for <bs>_

Even though underline is strictly what this should be. <bs>_ was used by
nroff to indicate italics which wasn't possible on old typewriters so
underline was used. Modern terminals now support italics so lets use
that now.

See:
- https://unix.stackexchange.com/questions/274658/purpose-of-ascii-text-with-overstriking-file-format/274795#274795
- https://cmd.inp.nsk.su/old/cmd2/manuals/unix/UNIX_Unleashed/ch08.htm

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())`.

The environment variable CIRRUS_CI is manually passed to RunTests.cmake
as it doesn't get passed when using cmake script mode.

* fix(man): handle absolute paths as :Man targets

Previously, attempting to provide `:Man` with an absolute path as the name would
cause neovim to return the following error:

```
Error detected while processing command line:
/usr/local/share/nvim/runtime/lua/man.lua:690: /usr/local/share/nvim/runtime/lua/man.lua:683: Vim:E426: tag not found: nil(nil)
Press ENTER or type command to continue
```

..because it would try to validate the existence of a man page for the provided
name by executing `man -w /some/path` which (on at least some Linux machines
[0]) returns `/some/path` instead of the path to the nroff files that would be
formatted to satisfy the man(1) lookup.

While man pages are not normally named after absolute paths, users shouldn't be
blamed for trying. Given such a name/path, neovim would **not** complain that
the path didn't have a corresponding man file but would error out when trying
to call the tag function for the null-propagated name-and-section `nil(nil)`.
(The same underlying error existed before this function was ported to lua, but
did not exhibit the lua-specific `nil(nil)` name; instead a tag lookup for `()`
would fail and error out.)

With this patch, we detect the case where `man -w ...` returns the same value as
the provided name to not only prevent invoking the tag function for a
non-existent/malformed name+sect but also to properly report the non-existence
of a man page for the provided lookup (the absolute path).

While man(1) can be used to directly read an nroff-formatted document via `man
/path/to/nroff.doc`, `:Man /path/to/nroff.doc` never supported this behavior so
no functionality is lost in case the provided path _was_ an nroff file.

[0]: `man -w /absolute/path` returning `/absolute/path` observed on an Ubuntu
18.04 installation.

* test: add regression test for #20624

Add a functional test to `man_spec.lua` to check for a regression for #20624 by
first obtaining an absolute path to a random file and materializing it to disk,
then attempting to query `:Man` for an entry by that same name/path.

The test passes if nvim correctly reports that there is no man page
correspending to the provided name/path and fails if any other error (or no
error) is shown.

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