[illumos/solaris] set MSG_NOSIGNAL while writing to sockets

Both these platforms have MSG_NOSIGNAL available, and we should set it for
socket writes in the event that the SIGPIPE handler has been reset to SIG_DFL
(i.e. terminate the process).

I've verified via a quick program at
https://github.com/sunshowers/msg-nosignal-test/ that even when the SIGPIPE
handler is reset to SIG_DFL, writes to closed sockets now error out with EPIPE.
(Under ordinary circumstances UDP writes won't cause MSG_NOSIGNAL.)

library/std/src/os/unix/net/datagram.rs

@@ -20,6 +20,8 @@
     target_os = "freebsd",
     target_os = "openbsd",
     target_os = "netbsd",
+    target_os = "solaris",
+    target_os = "illumos",
     target_os = "haiku",
     target_os = "nto",
 ))]
@@ -31,6 +33,8 @@
     target_os = "freebsd",
     target_os = "openbsd",
     target_os = "netbsd",
+    target_os = "solaris",
+    target_os = "illumos",
     target_os = "haiku",
     target_os = "nto",
 )))]

library/std/src/sys_common/net.rs

@@ -42,6 +42,7 @@
         target_os = "hurd",
         target_os = "dragonfly", target_os = "freebsd",
         target_os = "openbsd", target_os = "netbsd",
+        target_os = "solaris", target_os = "illumos",
         target_os = "haiku", target_os = "nto"))] {
         use libc::MSG_NOSIGNAL;
     } else {