Exclusively use OAI_RNGSEED env variable everywhere

Hide normal table initialization of Ziggurat generator from outside to
force the usage of randominit() for RNG initialization. In this step,
remove the randominit() seed parameter and only rely on OAI_RNGSEED for
a consistent RNG initialization, as this parameter was used differently
("0" as fixed number or to force a random seed).

Now, only use OAI_RNGSEED env variable to force the usage of a fixed
seed.

doc/environment-variables.md

@@ -9,7 +9,8 @@ OAI uses/supports a number of environment variables, documented in the following
 - `NVRAM_DIR`: directory to read/write NVRAM data in (5G) `nvram` tool; if not defined, will use `PWD` (working directory)
 - `OAI_CONFIGMODULE`: can be used to pass the configuration file instead of `-O`
 - `OAI_GDBSTACKS`: if defined when hitting an assertion (`DevAssert()`, `AssertFatal()`, ...), OAI will load `gdb` and provide a backtrace for every thread
-- `OAI_RNGSEED`: overwrites any seed for random number generators (RNG) in simulators
+- `OAI_RNGSEED`: provide a fixed seed for random number generators (RNG). If
+  this variable is absent, a random seed will be read (from `/dev/urandom`).
 - `USIM_DIR`: directory to read/write USIM data in (4G) `usim` tool; if not defined, will use `PWD` (working directory)
 
 Furthermore, these variables appear in code that is not maintained and maybe not even compiled anywhere: