macro: const'ify MIN/MAX/... macros

We must add 'const' to local variables in statement-expressions to
guarantee that the macros can produce constant-expressions if given such.
GCC seems to ignore this, but LLVM/clang requires it (understandably).
macro: add CONST_MAX() macro

The CONST_MAX() macro is similar to MAX(), but verifies that both
arguments have the same type and are constant expressions. Furthermore,
the result of CONST_MAX() is again a constant-expression.

CONST_MAX() avoids any statement-expressions and other non-trivial
expression-types. This avoids rather arbitrary restrictions in both GCC
and LLVM, which both either fail with statement-expressions inside
type-declarations or statement-expressions inside static-const
initializations.

If anybody knows how to circumvent this, please feel free to unify
CONST_MAX() and MAX().
resolve: fix compilation on LLVM+clang

LLVM+clang does not allow statement-expressions inside of
type-declarations (file-scope). Use CONST_MAX() to avoid this.
hostnamectl: actually implement location support