Fix return values of recv/send functions

Fix return values of recv/send functions

In the current headers the recv and send class functions return an int.
However, according to the man pages they should return a ssize_t.

Signed-off-by: Marc Rittinghaus <marc.rittinghaus@kit.edu>
Reviewed-by: Cezar Craciunoiu <cezar.craciunoiu@gmail.com>
Reviewed-by: Razvan Deaconescu <razvan.deaconescu@cs.pub.ro>
Approved-by: Razvan Deaconescu <razvan.deaconescu@cs.pub.ro>
Tested-by: Unikraft CI <monkey@unikraft.io>
GitHub-Pull-Request: #17