doc: remove legacy os_*.txt

keeping os_dos.txt, see #1115
doc: remove gui_w16.txt
doc: remove if_sniff.txt
doc: remove if_ole.txt
doc: remove workshop.txt
doc: remove netbeans.txt
doc: remove documentation for `:shell`

see #450
doc: cleanup

  * fix some links
  * typo
  * remove reference to hangul_input
  * remove unused vim2html.pl
doc: remove links to non existent files
Merge pull request #1268 from stefan991/cleanup-docs-2

Cleanup user documentation