fixes #10295

(less) 2> /dev/null would fail if stdin is /dev/null.
Sinceless --version do not read anything from stdin,
it is perfect for seeing if the software exist or not.

Also replace the whole os system detection by directly
using subprocess ( as we use it elsewhere, we already
depend on it ).

This can be used from another non interactive software, see #9695
for details.

more intelligent about ignoring files that are clearly not modules

function

added display of deprecated to ansible-doc
now it does better job of using tty columns
fixed indication truncation of desc with trailing ...
removed extension from module list, also fixed matching exlusion
blacklist

'_', they will be loaded after non prefixed modules are checked they can
be full modules or symlinks to existing ones (alias)

also updated ansible doc to ignore these, will eventually add selective
display

When running `ansible-doc --list` a lot of errors are
generated when `ansible-doc` tries to find documentation
strings in `.pyc` files.

Increases readability when using `ansible-doc`.

amend ansible-doc to include default options as per feature request #7938

skip "module" not found as plugin, id est directory

If PAGER is set, or the executable less is present, ansible-doc will use
it to pipe information into so that it can be scrolled through.
If the environment variable LESS is not set, this will set it to FRSX.

And also be liberal in what ansible-doc accepts as an empty
requirements specification.

While here fix a typo in template module documentation
wrt jinja2 overrides example.

tweak position & look of Requirements in rst
tweak APT's notes as per anhj's wish

return DOC and EXAMPLES as a list
add moduledev explanation
more

check path is directory
added manpage & setup
small cleanup
shut up module_formatter in utils to avoid trace print on crud files in library