Use modern C and modern compiler features [v9.18]

This MR consists of several commits that introduce modern C or modern compiler features (with fallbacks):

• Gcc 7+/Clang 10+ have __attribute__((__fallthrough__)), use that as FALLTHROUGH; macro
• Use __builtin_unreachable() for ISC_UNREACHABLE(); macro and merge INSIST(0); ISC_UNREACHABLE(); just to UNREACHABLE();
• Use C11 noreturn with fallback to __attribute__((__noreturn__))
• Remove use of the inline keyword used as a suggestion to the compiler
• Remove workarounds for ancient versions of clang (<< 4.0.1)

Backport of !5480 (merged)