Send, sendto, recv, recvfrom syscall comments and tests (#314)

* Initial comments on send syscall

* Small comments

* More small changes

* Added two tests for send sys call and removed MSG_NOSIGNAL flag checking

* updated sendto func

* Comments on recv helper functions

* Unix send/recv test

* updates from qianxi and vlad

* Small comment

* minor touches

* Small changes in style

* Changed panic to syscall error

* updated errno formatting

---------

Co-authored-by: Nicholas Renner <nicholassrenner@gmail.com>

Author: davidge20