[PATCH] Use strchr() to search for a single character

Code in src/port/pgmkdirp.c uses strstr() to find a single character in
a string, but strstr() seems to be too generic for this job. Another
function, strchr(), might be better suited for this purpose, because it
is optimized to search for exactly one character in a string. In
addition, if strchr() is used, the compiler doesn't have to generate a
terminating \0 byte for the substring, producing slightly smaller code.
I'm attaching the patch.

Regards,
Dmitry