daemon: use __attribute__((constructor)) for protolayer_globals
The protolayer_globals array can basically be treated as a constant by
most of the program and its initialization only uses compile-time-known
values. We basically only initialize parts of the array in different
files throughout the codebase to maintain separation of concerns, so
that each piece of Knot Resolver initializes the part that pertains to
it. Therefore, I believe that it is more ergonomic to just use
__attribute__((constructor)) for these functions, so as not to pollute
daemon/main.c with these calls.
