runtime: Change external plugin directory to "external-plugin"
runtime: Prepend the vimrc filename to the external plugin manifest

This is required to support multiple vimrcs with each having it's own set of
installed external plugins.
doc: Update external plugin documentation
doc: Add nvim_provider/external_plugin to doc Makefile
Merge PR #1504 'Change external plugin dir, manifest and improve docs'
Remove win16 references
Remove NeXTSTEP references
Remove Atari MiNT references
Remove OS/2 references

NOTE: Paul Slootman was removed from the top of os_unix.c but is still
credited somewhere in the docs dir...
Remove Windows 95 references
Remove BeOS/Haiku references