Special radio channels now use bitflags instead of individual vars (#85150)

## About The Pull Request

Radios/encryption keys now use a `RADIO_SPECIAL_XXXXX` bitflag for
behaviors related to "special" radio channels (Binary, Centcom, Syndie).

I decided to do this because I wanted to add a radio channel for pirates
and hunters (hence the branch name), but it felt weird adding two more
variables. The more I look at the changes I've made here the more I
realize that the effort was probably not worth the utility but whatever.

This also subtypes some varedited intercoms and makes them their own
objects.
## Why It's Good For The Game

Compresses a whopping three (3!) variables into a single one. Easier to
scale (I guess?). I felt like adding a fourth/fifth variable and just
moving on with the original project, but decided "lets do this the
unnecessarily hard way instead".
## Changelog
🆑 Rhials
code: Radios/encryption keys now use a single variable for "special"
frequencies. Please report if you experience any strangeness with
accessing/being unable to access the Centcom, Syndicate, or Cyborg
radio.
/🆑

---------

Co-authored-by: Ghom <42542238+Ghommie@users.noreply.github.com>

code/modules/unit_tests/say.dm

@@ -237,9 +237,9 @@
 	// Normally speaking, if there isn't a functional telecomms array on the same z-level, then handheld radios
 	// have a short delay before sending the message. We use the centcom frequency to get around this.
 	speaker_radio.set_frequency(FREQ_CENTCOM)
-	speaker_radio.independent = TRUE
+	speaker_radio.special_channels = RADIO_SPECIAL_CENTCOM
 	listener_radio.set_frequency(FREQ_CENTCOM)
-	listener_radio.independent = TRUE
+	listener_radio.special_channels = RADIO_SPECIAL_CENTCOM
 
 	var/pangram_quote = "The quick brown fox jumps over the lazy dog"