zephyr_login refactoring

* Add `login_tzc` and `login_zeph02` functions to remove redundant comparison
* Add `get_zephyr_realm` function to reduce code duplication
* Extract some functions
* Use `FALSE` in `purple_account_get_bool`
* Replace `read_anyone` and `read_zsubs` variables with functions calls

Testing Done:
Compile.

Reviewed at https://reviews.imfreedom.org/r/405/