HIGH DEF-P0-7: relibc MSG_NOSIGNAL now properly blocks SIGPIPE via
pthread_sigmask around the syscall, instead of stripping the flag.
sendmsg does the sigprocmask block; sendto forwards the original
flags to sendmsg (removing the previous 'flags & !MSG_NOSIGNAL'
workaround).
Also removed the stale 'TCP lacks SocketCall::SendMsg handling' TODO
- the netstack scheme handler at
local/sources/base/netstack/src/scheme/socket.rs:519-533 does handle
SocketCall::SendMsg for both SOCK_STREAM and SOCK_DGRAM.
vasilito
d372eb0169