[clang][analyzer] Improve StdCLibraryFunctions socket send/recv functions.

The modeling of send, recv, sendmsg, recvmsg, sendto, recvfrom is changed:
These functions do not return 0, except if the message length is 0.
(In sendmsg, recvmsg the length is not checkable but it is more likely
that a message with 0 length is invalid for these functions.)

Reviewed By: donat.nagy

Differential Revision: https://reviews.llvm.org/D155715