Move tzc and zeph02 to own modules and some cleanup

* Add `send_message`, `set_location`, `get_subs_from_server`, `close` to `zephyr_account`
* Remove now unused `use_tzc` and `use_zeph02`
* Format message inside `zephyr_write_message`
* Rename some functions
* Reduce scope for some variables
* Remove unused `ZEPHYR_FD_*` macros

Testing Done:
Compile.

Reviewed at https://reviews.imfreedom.org/r/471/