[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

common/src/stringutils.c

@@ -23,6 +23,7 @@
 #include <stdlib.h>
 #include <stdarg.h>
 #include <string.h>
+#include <errno.h>
 
 #include "common/stringutils.h"
 
@@ -89,3 +90,23 @@ bool str_containsValue(const char * list, char delimiter, const char * value)
   }
   return false;
 }
+
+char * strdup(const char *s)
+{
+  if (!s)
+  {
+    errno = EINVAL;
+    return NULL;
+  }
+
+  const ssize_t len = strlen(s) + 1;
+  char * out = malloc(len);
+  if (!out)
+  {
+    errno = ENOMEM;
+    return NULL;
+  }
+
+  memcpy(out, s, len);
+  return out;
+}