Removes shelltype option and all revelant doc. #1249

Fixes #1240
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
memory: implement xmemrchr()
call_shell: Add callback param to os_call_shell().

Change `read_cb()` to execute the user callback and to ensure the input
is line-wise.

Factor `do_read_cb()` out of `read_cb()` and make it the default
callback for `os_call_shell()` so that `kShellOptRead` works as
expected.

Use NULLs for the callbacks at each call site so they can be update, one
by one.
call_shell: avoid copying in do_read_cb.
call_shell: Use a callback with get_cmd_output.
doxygen: do_shell()
call_shell: Add callback to do_shell()
call_shell: Use a callback for "diff" output.

Make read_diff() reentrant by locallizing its variables and storing the
ones that persist in a structure. Use it as a callback.
call_shell: Add input/input_len parameters.

Change write_selection() to save_selection().
call_shell: Remove unnecessary `reading` variable from `ProcessData`

See #980.