added Curl_strcasestr() for case insensitive strstr() searching

lib/strequal.c

@@ -78,6 +78,25 @@ int curl_strnequal(const char *first, const char *second, size_t max)
 #endif
 }
 
+/*
+ * Curl_strcasestr() finds the first occurrence of the substring needle in the
+ * string haystack.  The terminating `\0' characters are not compared. The
+ * matching is done CASE INSENSITIVE, which thus is the difference between
+ * this and strstr().
+ */
+char *Curl_strcasestr(const char *haystack, const char *needle)
+{
+  size_t nlen = strlen(needle);
+  size_t hlen = strlen(haystack);
+
+  while(hlen-- >= nlen) {
+    if(curl_strnequal(haystack, needle, nlen))
+      return (char *)haystack;
+    haystack++;
+  }
+  return NULL;
+}
+
 #ifndef HAVE_STRLCAT
 /*
  * The strlcat() function appends the NUL-terminated string src to the end

lib/strequal.h

@@ -36,6 +36,9 @@ int curl_strnequal(const char *first, const char *second, size_t max);
    argument is zero-byte terminated */
 #define checkprefix(a,b)    strnequal(a,b,strlen(a))
 
+/* case insensitive strstr() */
+char *Curl_strcasestr(const char *haystack, const char *needle);
+
 #ifndef HAVE_STRLCAT
 #define strlcat(x,y,z) Curl_strlcat(x,y,z)
 size_t Curl_strlcat(char *dst, const char *src, size_t siz);