Fix CppCheck style errors

#if 0 the content of toxav/msi.c : int stringify_message(MSIMessage
*msg, char *dest)
This function has no effect and does not seem to be used for actively
waiting.

Fix various other style errors, reduce scope when possible, avoid
redundant writes, clarify operator priorities, etc.
Got a little dated
Fix FreeBSD instructions, sodium 0.5.0

Sodium 0.5.0 optimizes itself on being loaded
Update README.md
Merge branch 'tux3-cppcheck-style'
Merge branch 'patch-1' of https://github.com/stqism/ToxCore