misc/mbuffer: Update to 20200929

Upstream changes according to the ChangeLog:

• enhancement: added option --no-direct to disable use of O_DIRECT
• defaults: raised default TCP timeout to 10ms for WAN connections
• fix: leave TCP buffer size untouched if not set
• enhancement: add option to set TCP timeout
• performance optimization: use recv with MSG_WAITALL instead of read
• configure fix: look for objdump also with target prefix
• testing: make sure to use gtar for testing to avoid unexpected failures
• portability: NetBSD compatibility fix
• build enhancement: added dependency calculation for make
• build enhancement: automatic version string generation

PR: 252737
Submitted by: Fabian Keil <fk@fabiankeil.de>