Clarify documentation & add function to clear global logging settings. And implement correct usage in tools.

Get rid of warnings in fastpkdf.c, disable -Watomic-implicit-seq-cst as it seems borked on FreeBSD
Add FreeBSD CI

Also remove dead code in rist-common.c, it was being optimized away anyway.

The RIST committee in all it's wisdom decided to use some of the reserved bits for the version, instead of the existing GRE version field (likely to keep somewhat of compatibility with existing GRE implementations).

"RFC 8086 defines bits 4-12 in the GRE header as Reserved0.  This Specification defines bits 9-12 as follows (see Figure 1):
o   RV (bits 10-12): this field indicates the RIST Version, as follows:
§  RV = 000: TR-06-2:2020
§  RV = 001: TR-06 -2-2021
o   H (bit 9): this field indicates the AES key length for PSK operation, as follows:
§  H = 0: 128-bit AES key
§  H = 1: 256-bit AES key
·         Usage of the H bit shall be as follows:
o   If RV = 000, the H bit shall be set to zero on transmission and ignored on reception.  In this case, the PSK key length shall be determined by out-of-band means.
o   If RV = 001 or higher, the H bit indicates the PSK key length.  Devices not operating in PSK mode shall set the H bit to zero on transmission and ignore it on reception.
·         The remaining Reserved0 bits in Figure 1 shall be set to zero on transmission and ignored on reception, as per RFC 2784."

With this method, we are only importing the mbedcrypto library files and are
compiling them using meson/ninja direclty inside our main project.
This seems to solve all the cross-compiling issues and has no downside.
For the systems that already have the library installed, the compile
script will use that one instead by default.

Fixes #95 and #96

Fix peer and flow timeout checks. They were incorrectly being marked as dead when  the configured buffer was smaller than the RTCP intervals.