This breaks the existing libathemecore/arc4random.c down into 3 files:

- libathemecore/random_frontend.c

  This file is responsible for including one of the frontends below.
  This inclusion approach is elucidiated in commit bf697f05.

- libathemecore/random_fe_internal.c

  This frontend is the internal ChaCha20-based fallback RNG taken from
  OpenBSD libc. It is used if no other files below are suitable.

- libathemecore/random_fe_mbedtls.c

  This frontend is the RNG based on ARM mbedTLS HMAC-DRBG. This is just
  the code that was already in libathemecore, but in another file
  instead of intertwined with ifdef spaghetti.

Additionally, 2 new frontends are introduced:

- libathemecore/random_fe_openbsd.c

  This frontend is used if building on an OpenBSD machine. It uses
  OpenBSD's arc4random(3), located in libc.

- libathemecore/random_fe_sodium.c

  This frontend uses the randombytes API in libsodium.

The preferred frontend order is:

  - OpenBSD arc4random(3), or
  - libsodium randombytes(3), or
  - ARM mbedTLS hmac_drbg_random(3), or
  - Internal ChaCha20-based Fallback RNG, seeded by:
    - getentropy(3), or
    - getrandom(2), or
    - urandom(4)