Add detection for headers which are included portably, and remove them.

Cast printf argument of type time_t to long long to match the %lld format specifier on platforms where this might not be the case.

This introduces an area for helper scripts when dealing with -portable syncing from upstream, releases, etc. See the README.md file for further information.

This is included portably.

Reminded by naddy and stsp; it was missing a cast to unsigned char to prevent issues on archs with signed chars and was too broad anyway. While here, drop an extra check immediately after. ok stsp@

Don't rely on implicit/"hidden" includes, but rather add them to the header file so their inclusion doesn't result in missing values. Although not explicitly a functional change, this does help -portable. This ensures size_t/time_t are always present. OK @jrick

spotted by op@

This is included portably.