Assume STDC_HEADERS will always be defined

Assume STDC_HEADERS will always be defined

We're in 2021.  C89 is everywhere; in fact, there are many other
assumptions in the code that wouldn't probably hold on
pre-standard C environments.  Let's simplify and assume that C89
is available.

The specific assumptions are that:
- <string.h>, and <stdlib.h> are available
- strchr(3), strrchr(3), and strtok(3) are available
- isalpha(3), isspace(3), isdigit(3), and isupper(3) are available

I think we can safely assume we have all of those.

Signed-off-by: Alejandro Colomar <alx.manpages@gmail.com>