Extend select+rlimit sanbox test to include poll.

POSIX specifies that poll() shall fail if "nfds argument is greater
than {OPEN_MAX}".  The setrlimit sandbox sets this to effectively zero
so this causes poll() to fail in the preauth privsep process.

This is likely the underlying cause for the previously observed similar
behaviour of select() on plaforms where it is implement in userspace on
top of poll().

configure.ac

@@ -3518,10 +3518,11 @@ AC_ARG_WITH([sandbox],
 	]
 )
 
-# Some platforms (seems to be the ones that have a kernel poll(2)-type
-# function with which they implement select(2)) use an extra file descriptor
-# when calling select(2), which means we can't use the rlimit sandbox.
-AC_MSG_CHECKING([if select works with descriptor rlimit])
+# POSIX specifies that poll() "shall fail with EINVAL if the nfds argument
+# is greater than OPEN_MAX".  On some platforms that includes implementions
+# ofselect in userspace on top of poll() so check both work with rlimit NOFILES
+# so check that both work before enabling the rlimit sandbox.
+AC_MSG_CHECKING([if select and/or poll works with descriptor rlimit])
 AC_RUN_IFELSE(
 	[AC_LANG_PROGRAM([[
 #include <sys/types.h>
@@ -3532,6 +3533,11 @@ AC_RUN_IFELSE(
 #ifdef HAVE_SYS_SELECT_H
 # include <sys/select.h>
 #endif
+#ifdef HAVE_POLL_H
+# include <poll.h>
+#elif HAVE_SYS_POLL_H
+# include <sys/poll.h>
+#endif
 #include <errno.h>
 #include <fcntl.h>
 #include <stdlib.h>
@@ -3540,6 +3546,9 @@ AC_RUN_IFELSE(
 	int fd, r;
 	fd_set fds;
 	struct timeval tv;
+#ifdef HAVE_POLL
+	struct pollfd pfd;
+#endif
 
 	fd = open("/dev/null", O_RDONLY);
 	FD_ZERO(&fds);
@@ -3550,7 +3559,16 @@ AC_RUN_IFELSE(
 	tv.tv_sec = 1;
 	tv.tv_usec = 0;
 	r = select(fd+1, &fds, NULL, NULL, &tv);
-	exit (r == -1 ? 1 : 0);
+	if (r == -1)
+		exit(1);
+#ifdef HAVE_POLL
+	pfd.fd = fd;
+	pfd.events = POLLIN;
+	r = poll(&pfd, 1, 1);
+	if (r == -1)
+		exit(2);
+#endif
+	exit(0);
 	]])],
 	[AC_MSG_RESULT([yes])
 	 select_works_with_rlimit=yes],