runtime: don't execute external commands when loading ftplugins

runtime: don't execute external commands when loading ftplugins

This is a followup to 816fbcc262687b81fc46f82f7bbeb1453addfe0c (patch
9.0.1833: [security] runtime file fixes)

It basically disables that external commands are run on loading of the
filetype plugin, **unless** the user has set the `g:plugin_exec = 1`
global variable in their configuration or for a specific filetype the
variable g:<filetype>_exec=1.

There are a few more plugins, that may execute system commands like
debchangelog, gitcommit, sh, racket, zsh, ps1 but those do at least
do not run those commands by default during loading of the filetype plugin
(there the command is mostly run as convenience for auto-completion or
to provide documentation lookup).

closes: #13034

Signed-off-by: Christian Brabandt <cb@256bit.org>
Co-authored-by: Tim Pope <vim@tpope.org>