kqueue tests: fix -fno-common build

vnode_fd and kqfd are both shared among multiple CU; define them exactly
once.

In the case of vnode_fd, it was simply the declaration that needed
correction.

-fno-common will become the default in GCC10/LLVM11.

MFC after:	3 days

tests/sys/kqueue/libkqueue/common.h

@@ -41,7 +41,8 @@
 #include <sys/event.h>
 
 extern char *cur_test_id;
-int vnode_fd;
+extern int vnode_fd;
+extern int kqfd;
 
 extern char * kevent_to_str(struct kevent *);
 struct kevent * kevent_get(int);

tests/sys/kqueue/libkqueue/proc.c

@@ -25,7 +25,6 @@
 
 static int sigusr1_caught = 0;
 
-int kqfd;
 
 static void
 sig_handler(int signum)

tests/sys/kqueue/libkqueue/read.c

@@ -18,7 +18,6 @@
 
 #include "common.h"
 
-int kqfd;
 int sockfd[2];
 
 static void

tests/sys/kqueue/libkqueue/signal.c

@@ -18,7 +18,6 @@
 
 #include "common.h"
 
-int kqfd;
 
 void
 test_kevent_signal_add(void)

tests/sys/kqueue/libkqueue/timer.c

@@ -26,7 +26,6 @@
 #define	MS_TO_US(t)  ((t) * THOUSAND)	/* Convert milliseconds to microseconds. */
 #define	US_TO_NS(t)  ((t) * THOUSAND)	/* Convert microseconds to nanoseconds. */
 
-int kqfd;
 
 /* Get the current time with microsecond precision. Used for
  * sub-second timing to make some timer tests run faster.

tests/sys/kqueue/libkqueue/user.c

@@ -18,7 +18,6 @@
 
 #include "common.h"
 
-int kqfd;
 
 static void
 add_and_delete(void)

tests/sys/kqueue/libkqueue/vnode.c

@@ -18,7 +18,6 @@
 
 #include "common.h"
 
-int kqfd;
 int vnode_fd;
 
 void