Conditional buffering in 1.4.764

Version 1.4.764 has been released.
size(), replace() and substr() functions are now available for string manipulation. Also added buffer_size() and buffer_count() functions to the pm_buffer module. This allows conditional buffering, see the Explicit drop section in the reference manual and this mailing list post about how this can be used.
Some other issues and bugs were fixed, details are in the ChangeLog.