Check for -- and remove -- some unnecessarily included header files.

Check for -- and remove -- some unnecessarily included header files.

* Makefile.maint: Add checks for a handful of additional header files.
* src/mktemp.c: Don't include "long-options.h".  Not used.
* src/pr.c: Don't include "inttostr.h".  Not used.
* src/printenv.c: Don't include "error.h".  Not used.
* src/test.c: Don't include "error.h".  Not used.
* src/touch.c: Don't include "safe-read.h".  Not used.