Merge lib/dns/gen.h contents to lib/dns/gen.c (v9.18)
Formerly, the gen.h header contained a compatibility layer between Win32 and POSIX platforms. Since we have already dropped the Win32 build, we can merged gen.h into gen.c as the header file is not used elsewhere.
Backport of !5755 (merged)
Edited by Michał Kępień