Add the poll() syscall as permitted by the sandbox

Apparently, sometimes getpwnam will call this.

Fixes bug 25513.

changes/feature25313

@@ -0,0 +1,4 @@
+  o Minor features (sandbox):
+    - Explicitly permit the poll() system call when the Linux seccomp2-based
+      sandbox is enabled: apparently, some versions of libc use poll() when
+      calling getpwnam(). Closes ticket 25313.

src/common/sandbox.c

@@ -247,7 +247,8 @@ static int filter_nopar_gen[] = {
     SCMP_SYS(recvmsg),
     SCMP_SYS(recvfrom),
     SCMP_SYS(sendto),
-    SCMP_SYS(unlink)
+    SCMP_SYS(unlink),
+    SCMP_SYS(poll)
 };
 
 /* These macros help avoid the error where the number of filters we add on a