Use portable variadic macros for debugging

Switch from GNU-specific to portable, standards-conforming variadic
macros. Should silence _many_ clang warnings when using -pedantic.
Print correct elements when receiving peer header

The generic header has a uint8_t type and an array of three
uint8_t in variable "a". Instead of printing type + a[0-2],
I was erroneously printing a[0-3]. This was caught by clang.
Don't inline publicly visible functions

Clang complains that it cannot find them at link time.