[issue #405] updating NIST 800-53 refs for accounts_password_warn_age

[shawndwells]

resolves #405

AC-2(2) language:

(2) ACCOUNT MANAGEMENT | REMOVAL OF TEMPORARY / EMERGENCY ACCOUNTS
The information system automatically [Selection: removes; disables] temporary and emergency
accounts after [Assignment: organization-defined time period for each type of account].
Supplemental Guidance: This control enhancement requires the removal of both temporary and
emergency accounts automatically after a predefined period of time has elapsed, rather than at
the convenience of the systems administrator.

Feedback from NRO / @lukek1 & @lindelled's program request this mapping. This also removes discrepancies between SSG mappings and those provided by SECSCAN

[iankko]

Just out-of-curiosity wouldn't rules:
Set Account Expiration Following Inactivity (account_disable_post_pw_expiration)
Assign Expiration Date to Temporary Accounts (account_temp_expire_date)

wouldn't be better candidate rules to hold the AC-2(2) label? (but after check looks these two rules contain that label already).

So adding it to accounts_password_warn_age is just an enhancement. No objections for that (ACK wrt to patch syntax correctness).