Cleanup: use of the term 'len' & 'maxlen'

Only use the term len & maxlen when they represent the length & maximum
length of a string. Instead of the available bytes to use.

Also include the data they're referencing as a suffix, otherwise it's
not always clear what the length is in reference to.

source/blender/python/generic/py_capi_utils.h

@@ -21,7 +21,7 @@ void PyC_ObSpit(const char *name, PyObject *var);
  * A version of #PyC_ObSpit that writes into a string (and doesn't take a name argument).
  * Use for logging.
  */
-void PyC_ObSpitStr(char *result, size_t result_len, PyObject *var);
+void PyC_ObSpitStr(char *result, size_t result_maxncpy, PyObject *var);
 void PyC_LineSpit(void);
 void PyC_StackSpit(void);
 PyObject *PyC_ExceptionBuffer(void);