errors when setting filetype in modeline

Hi,

I get errors from vim in files that set the filetype through modeline.

Reproducer: A single file with only one line
```
# vim:ft=perl
```

upon opening vim, these error messages get printed:
```
"/tmp/test" 1L, 14C
Error detected while processing /usr/share/vim/vim81/ftplugin/perl.vim:
line 18:
E523: Not allowed here: keywordprg=perldoc\ -f
line 50:
E12: Command not allowed from exrc/vimrc in current dir or tag search
Error detected while processing function <SNR>13_LoadFTPlugin:
line 17:
E170: Missing :endfor
```

When manipulating the line, such that it is not a valid modeline, I see that filetype gets set to conf by:
```
filetype=conf
Last set from /usr/share/vim/vim81/filetype.vim line 2134
```

I see this behaviour in the vim from arch linux without `.vimrc` file or `.vim` directory.

```
prompt> vim --version
VIM - Vi IMproved 8.1 (2018 May 18, compiled Nov 22 2018 19:40:14)
Included patches: 1-542
Compiled by Arch Linux
Huge version without GUI. Features included (+) or not (-):
+acl +extra_search +mouse_netterm +tag_old_static
+arabic +farsi +mouse_sgr -tag_any_white
+autocmd +file_in_path -mouse_sysmouse +tcl/dyn
+autochdir +find_in_path +mouse_urxvt +termguicolors
-autoservername +float +mouse_xterm +terminal
-balloon_eval +folding +multi_byte +terminfo
+balloon_eval_term -footer +multi_lang +termresponse
-browse +fork() -mzscheme +textobjects
++builtin_terms +gettext +netbeans_intg +timers
+byte_offset -hangul_input +num64 +title
+channel +iconv +packages -toolbar
+cindent +insert_expand +path_extra +user_commands
-clientserver +job +perl/dyn +vartabs
-clipboard +jumplist +persistent_undo +vertsplit
+cmdline_compl +keymap +postscript +virtualedit
+cmdline_hist +lambda +printer +visual
+cmdline_info +langmap +profile +visualextra
+comments +libcall +python/dyn +viminfo
+conceal +linebreak +python3/dyn +vreplace
+cryptv +lispindent +quickfix +wildignore
+cscope +listcmds +reltime +wildmenu
+cursorbind +localmap +rightleft +windows
+cursorshape +lua/dyn +ruby/dyn +writebackup
+dialog_con +menu +scrollbind -X11
+diff +mksession +signs -xfontset
+digraphs +modify_fname +smartindent -xim
-dnd +mouse +startuptime -xpm
-ebcdic -mouseshape +statusline -xsmp
+emacs_tags +mouse_dec -sun_workshop -xterm_clipboard
+eval +mouse_gpm +syntax -xterm_save
+ex_extra -mouse_jsbterm +tag_binary
system vimrc file: "/etc/vimrc"
user vimrc file: "$HOME/.vimrc"
2nd user vimrc file: "~/.vim/vimrc"
user exrc file: "$HOME/.exrc"
defaults file: "$VIMRUNTIME/defaults.vim"
fall-back for $VIM: "/usr/share/vim"
Compilation: gcc -c -I. -Iproto -DHAVE_CONFIG_H -D_FORTIFY_SOURCE=2 -march=x86-64 -mtune=generic -O2 -pipe -fstack-protector-strong -fno-plt -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=1
Linking: gcc -L. -Wl,-O1,--sort-common,--as-needed,-z,relro,-z,now -fstack-protector -rdynamic -Wl,-export-dynamic -Wl,-E -Wl,-rpath,/usr/lib/perl5/5.28/core_perl/CORE -Wl,-O1,--sort-common,--as-needed,-z,relro,-z,now -L/usr/local/lib -Wl,--as-needed -o vim -lm -ltinfo -lelf -lnsl -lacl -lattr -lgpm -ldl -Wl,-E -Wl,-rpath,/usr/lib/perl5/5.28/core_perl/CORE -Wl,-O1,--sort-common,--as-needed,-z,relro,-z,now -fstack-protector-strong -L/usr/local/lib -L/usr/lib/perl5/5.28/core_perl/CORE -lperl -lpthread -ldl -lm -lcrypt -lutil -lc -L/usr/lib -ltclstub8.6 -ldl -lz -lpthread -lm
```

A friend could reproduce the errors on Arch.

I do *not* see the bahaviour on debian testing (vim 8.1, patches 1-320)

So this looks like a bug in vim / the runtime files to me though would
appreciate confirmation or hints what else is wrong.

Thanks in advance,
Paul

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