Rework the user input interface to make it more modular

Rework the user input interface to make it more modular

This is will provide an interface for other mechanisms to be used to
query the user for information, such as usernames, passwords, etc.

It has also been a goal to make it possible to query for all the
information in one call and not do it sequencially as before.

 [v5 - Ensure password prompt is only displayed if we should read
       from stdin ]

 [v4 - add a simple wrapper combining query_user_{init,add,exec}()
     - change disapproved &= syntax ]

 [v3 - Avoid the dynamic list, use a static list of QUERY_USER_NUMSLOTS
     - The list of query_user data is now a global variable
     - Replaced query_user_init() with query_user_clear()
     - Make query_user_add() a void function
     - Rebased against master/600dd9a16fc61 ]

 [v2 - Removed the QUERY_USER_FOREACH macro
     - Avoided using underscore prefix in function names
     - Make query_user_init() do M_FATAL and become a void function
       instead of returning false in these unlikely situations ]

Signed-off-by: David Sommerseth <davids@openvpn.net>
Acked-by: Selva Nair <selva.nair@gmail.com>
Message-Id: 1472233732-27074-1-git-send-email-davids@openvpn.net
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg00137.html