Fixes when built with exotic headers

[chrysn]

Updating RIOT OS from Nimble 1.5.0 to 1.8.0, I found that some assumptions were made on header contents that are not backed by the specs:

• In a file using vnprintf, stdio.h was not included.
• In several files, a min() function was used without being defined -- presumably it is defined in some variations of the included headers, but not in one where it would be guaranteed.

The patches in this PR fix this; the implementation of min() was already present in several of the compilation units using the function, but not in all of them.