bug#66390: `man' allows to inject arbitrary shell code

[Maxim Nikulin <m.a.nikulin@gmail.com>]

man.el does not escape properly shell special characters when `man' is 
invoked with an argument to open particular manual page. As a result 
arbitrary shell code may be executed.

I do not consider it as a real issue when the `man' command is invoked 
by a user directly. However it is a security vulnerability when other 
packages calls `man' to open a specific page.

Consider an Org mode document with the following link and ol-man is loaded

   <man:File:\:UserDirs(3pm)>

In response to C-c C-o (`org-open-at-point') an error appears instead of 
formatted manual page

--- 8< ---
/usr/bin/sh: 1: Syntax error: "(" unexpected

process exited abnormally with code 2
--- >8 ---

Alternatively just evaluate

  (man "File:\\:UserDirs(3pm)")

A side note: I tried to add backslash due to an issue with ol-man that 
is to be fixed. A workaround in this particular case is to remove 
"(3pm)". Though the real problem is that special characters "()" are not 
quoted.

I would not consider the issue as a severe one unless some users who 
wish to open arbitrary Org files from the net

https://debbugs.gnu.org/cgi/bugreport.cgi?bug=58774#34
> Org files are native to Emacs, I wish to open Org files by using EWW.

man.el should prevent substitution of shell specials literally from 
`man' arguments into shell commands.