Design proposal to Non-Interactive password update for REST client

[vishwa]

This email is about openbmc/openbmc#1714 ( REST API to update root 
password )

Goal is to do Non-interactive password updates to enable a REST client 
to update the root password.

My proposal is to use `getspent(3)` and `putspent(3)` and here is the flow.

REST client will provide a method that takes std::string as parameter.

The Provider at the BMC will receive the password and does these:

  - Executes `getspent(3)` for "root" and gets the entries.
  - Parses the `sp_pwdp` and extracts `encryption method` , `salt`.
  - Makes a call to `crypt(3)` with the extracted `salt` and `user 
input` and generates encrypted pass-code
  - Populates the structure and calls `putspent(3)` to update the password

Please let me know your opinion on this.

Thank you,

!! Vishwa !!