[common] all: implement strdup directly

Dr.Memory on Windows complains bitterly about invalid heap free as
it doesn't seem to be able to track this function's allocations. As
it's such a trivial function we can just implement it locally.

common/include/common/stringutils.h

@@ -34,4 +34,7 @@ int alloc_sprintf(char ** str, const char * format, ...)
 // Find value in a list separated by delimiter.
 bool str_containsValue(const char * list, char delimiter, const char * value);
 
+// Local implementation of strdup
+char * strdup(const char *s);
+
 #endif