Introduce vfcntl()

This is a variant of fcntl that takes a va_list instead of variadic
arguments and can be used to safely interpose fcntl by libraries such
as epoll-shim that would otherwise have to reimplement the logic inside
fcntl.c to avoid reading incorrect arguments for CheriBSD.

See jiixyj/epoll-shim#36

lib/libsys/Symbol.sys.map

@@ -383,7 +383,8 @@ FBSD_1.7 {
 };
 
 FBSD_1.8 {
-	 kcmp;
+	kcmp;
+	vfcntl;
 };
 
 FBSDprivate_1.0 {

lib/libsys/fcntl.2

@@ -37,6 +37,9 @@
 .In fcntl.h
 .Ft int
 .Fn fcntl "int fd" "int cmd" "..."
+.In stdarg.h
+.Ft int
+.Fn vfcntl "int fd" "int cmd" "va_list ap"
 .Sh DESCRIPTION
 The
 .Fn fcntl

lib/libsys/fcntl.c

@@ -43,10 +43,12 @@
  * CHERI CHANGES END
  */
 
-#include <fcntl.h>
-#include <stdarg.h>
 #include <sys/types.h>
 #include <sys/syscall.h>
+
+#include <fcntl.h>
+#include <stdarg.h>
+
 #include "libc_private.h"
 
 typedef int (*fcntl_func)(int, int, intptr_t);
@@ -80,16 +82,24 @@ call_fcntl(fcntl_func fn, int fd, int cmd, va_list ap)
 	return (fn(fd, cmd, arg));
 }
 
+#pragma weak fcntl
+int
+vfcntl(int fd, int cmd, va_list ap)
+{
+	fcntl_func fn = (fcntl_func)*__libsys_interposing_slot(INTERPOS_fcntl);
+
+	return (call_fcntl(fn, fd, cmd, ap));
+}
+
 #pragma weak fcntl
 int
 fcntl(int fd, int cmd, ...)
 {
 	va_list args;
 	int result;
-	fcntl_func fn = (fcntl_func)*__libsys_interposing_slot(INTERPOS_fcntl);
 
 	va_start(args, cmd);
-	result = call_fcntl(fn, fd, cmd, args);
+	result = vfcntl(fd, cmd, args);
 	va_end(args);
 
 	return (result);

sys/sys/fcntl.h

@@ -376,6 +376,7 @@ int	open(const char *, int, ...);
 int	creat(const char *, mode_t);
 int	fcntl(int, int, ...);
 #if __BSD_VISIBLE
+int	vfcntl(int, int, __va_list);
 int	flock(int, int);
 int	fspacectl(int, int, const struct spacectl_range *, int,
 	    struct spacectl_range *);