Treat Hammerhead like Solaris/illumos in the os-layer meson build.

Meson derives host_machine.system() from uname sysname, which on Hammerhead
is "hammerhead", not "sunos". The os/ static library's Solaris branch (which
adds -lsocket/-lnsl and the _XOPEN_SOURCE/__EXTENSIONS__ defines needed for
struct msghdr.msg_control and the XPG socket API) therefore never fires,
producing undefined socket()/gethostbyname() symbols at link time. Hammerhead
IS illumos, so extend the guard to match its sysname.

--- a/os/meson.build
+++ b/os/meson.build
@@ -63,7 +63,7 @@
 os_c_args = []
 
 # eg. struct msghdr -> msg_control 
-if host_machine.system() == 'sunos'
+if host_machine.system() == 'sunos' or host_machine.system() == 'hammerhead'
     os_c_args += '-D_XOPEN_SOURCE=1'
     os_c_args += '-D_XOPEN_SOURCE_EXTENDED=1'
     os_c_args += '-D__EXTENSIONS__'
Fix inverted re-arm check in the illumos event-ports (PORT_SOURCE_FD) ospoll
backend - the listening socket only ever accepted exactly one client for the
lifetime of the server.

Root cause (found 2026-07-22, hh-alpha9, Task 8 M5 integration test):
illumos event ports are one-shot per delivered event - once port_getn()
returns an event for a PORT_SOURCE_FD object, that fd is automatically
dissociated from the port and MUST be re-associated (port_associate() again,
done here via epoll_mod()) to keep receiving further events on it.

ospoll_wait()'s PORT-backend dispatch loop re-arms level-triggered fds with:

    if (osfd->trigger == ospoll_trigger_level &&
        !xorg_list_is_empty(&osfd->deleted)) {
        epoll_mod(ospoll, osfd);
    }

osfd->deleted is the intrusive list node ospoll_remove() links onto
ospoll->deleted when an fd is torn down (see the xorg_list_add call right
after port_dissociate() in ospoll_remove()). So xorg_list_is_empty(&osfd->deleted)
is true exactly when this specific fd has NOT just been removed by its own
callback - i.e. it's still a live, valid fd safe to re-arm.

The listening socket is a level-triggered fd that lives for the entire
server lifetime and is never removed, so its deleted node is always empty.
With the `!` in front, the condition only fires when the fd HAS just been
deleted (backwards - re-arming an fd that's about to be freed), and never
fires for the common "still alive" case. Net effect: the listen socket's
single initial port_associate() (done at ospoll_add time) gets consumed by
the first accept() event and is never renewed, so every connection after
the first sits unnoticed in the kernel accept backlog forever.

Confirmed via truss -p <Xvfb pid>: after the first client's full connection
setup, port_getn() goes back to sleeping and no further accept(4, ...) call
is ever made, even though a second client's connect() has already completed
at the socket layer (visible in `netstat -f unix` as a live but un-accepted
pair). Fix: drop the negation so accepting fds get re-armed, and only skip
fds that were actually just torn down.

--- a/os/ospoll.c
+++ b/os/ospoll.c
@@ -638,7 +638,7 @@
             osfd->callback(osfd->fd, xevents, osfd->data);

         if (osfd->trigger == ospoll_trigger_level &&
-            !xorg_list_is_empty(&osfd->deleted)) {
+            xorg_list_is_empty(&osfd->deleted)) {
             epoll_mod(ospoll, osfd);
         }
     }