Problem:
If a manpage is opened, its hardwrapped dimensions are not recalculated
after closing then revisiting the same manpage.

Solution:
Unload the manpage when it is hidden. This forces it to be reloaded,
which forces the hard-wrapping to be recalculated when it is revisited.

Fixes: #25457

feat(lua): add vim.system()

Problem:

  Handling system commands in Lua is tedious and error-prone:
  - vim.fn.jobstart() is vimscript and comes with all limitations attached to typval.
  - vim.loop.spawn is too low level

Solution:

  Add vim.system().
  Partly inspired by Python's subprocess module
  Does not expose any libuv objects.

Current behaviour of `:Man` is to only work with "number" sections.
This is caused by wrong assumptions about man sections naming.

Also, there was similar assumption about length of section dirs
in `paths` variable.

fixes #23485

Signed-off-by: Vadim Misbakh-Soloviov <git@mva.name>

Fix #21169

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