mTCP 2020-03-07 Release Notes

This is a tiny release but there were some bug fixes that I didn't want to sit on, so here we are. You might want to look at the mTCP 2020-01-01 release notes for more context on what has changed recently.

Bug fixes:

• Check the malloc return codes in UDP.CPP; this was missed twice. (Thanks to Simon Howard for alerting me to this.)
• Pkttool needs to check before calling extended packet driver functions. (Thanks to Seth Simon for alerting me to this.)
• Spdtest crashes if MTU is less than 1500 bytes. (Also thanks to Seth for alerting me to this.)
  

Other improvements:

• Ping return codes are now more expressive. (Also thanks to Simon for requesting this.)
• Pkttool return codes are now documented.
• Spdtest dumps more statistics to help with performance debugging. It also warns about suboptimal MTU settings.