login.php: Use SessionVariables

Issue #99.
login.php: Refactor for consistent API

Issue #99.
login.php: Refactor for consistent API

Issue #99.
lib: Remote pip_append_session_to_array()

It's too specific to be useful and yet not accurately named.

Issue #99.
login.php: Make function comment more accurate

Issue #99.
lib: Abstract accounts functionality into accounts.php

Issue #99.
accounts.php: Refactor for consistent API

Issue #91.
accounts.php: Refactor for consistent API

Issue #99.
accounts.php: Refactor for consistent API
login.php: Refactor for consistent API

Issue #99.
login.php: Refactor for consistent API

Issue #99.
login.php: Refactor for consistent API

Issue #99.
login.php: Refactor for consistent API

Issue #99.
login.php: Refactor for consistent API

Issue #99.
login.php: Refactor for consistent API

Issue #99.
lib: Abstract registration functionality into register.php

Issue #99.
register.php: Refactor for consistent API

Issue #99.
post.php: Refactor for consistent API

Issue #99.
session.php: Refactor for consistent API

Issue #99.