Avoid core dumps for bogus ap_snprintf() format strings by using more

defensive approach: never allow patching a prefix char in front of
a constant string (or in front of char_buf); delimit strchr() to not
scan past the generated string;
Add 'h' modifier for compatibility reasons with other printf()s.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/1.3.x@81172 13f79535-47bb-0310-9956-ffa450edef68