Improved parsing of some integer arguments (sizes).

tools/brssl.h

@@ -163,6 +163,12 @@ typedef VECTOR(unsigned char) bvector;
  */
 int eqstr(const char *s1, const char *s2);
 
+/*
+ * Convert a string to a positive integer (size_t). Returned value is
+ * (size_t)-1 on error. On error, an explicit error message is printed.
+ */
+size_t parse_size(const char *s);
+
 /*
  * Structure for a known protocol version.
  */