str: provide checks for ctype.h char functions, and strstr and strchr functions.

In the former case, we were bitten by the fact that you don't pass a char
to isalpha() et al: you pass an int.  This means on most platforms you want
to do:

   if (isalpha((unsigned char)c)) ...

Insane?  Yes, but I assure you I'm not making this up.

Similarly, I've always wanted strstr, strchr and strrchr to return
const char * when given a const char * argument to search, to avoid
constness leak.

In both cases, the actual versions from the headers may be macros, so
we need to be very careful overriding them.  The result is that they
become out-of-line functions which is unacceptable for general
performance.

So we only activate these when CCAN_STR_DEBUG is defined.