use new SYS_faccessat2 syscall to implement faccessat with flags

commit 0a05eace163cee9b08571d2ff9d90f5e82d9c228 implemented AT_EACCESS
for faccessat with a horrible hack, creating a child process to change
switch uid/gid and perform the access probe without making potentially
irreversible changes to the caller's credentials. this was due to the
syscall lacking a flags argument.

linux 5.8 introduced a new syscall, SYS_faccessat2, fixing this
deficiency. use it if any flags are passed, and fallback to the old
strategy on ENOSYS. continue using the old syscall when there are no
flags.

src/unistd/faccessat.c

@@ -25,12 +25,17 @@ static int checker(void *p)
 
 int faccessat(int fd, const char *filename, int amode, int flag)
 {
-	if (!flag || (flag==AT_EACCESS && getuid()==geteuid() && getgid()==getegid()))
-		return syscall(SYS_faccessat, fd, filename, amode, flag);
+	if (flag) {
+		int ret = __syscall(SYS_faccessat2, fd, filename, amode, flag);
+		if (ret != -ENOSYS) return __syscall_ret(ret);
+	}
 
-	if (flag != AT_EACCESS)
+	if (flag & ~AT_EACCESS)
 		return __syscall_ret(-EINVAL);
 
+	if (!flag || (getuid()==geteuid() && getgid()==getegid()))
+		return syscall(SYS_faccessat, fd, filename, amode);
+
 	char stack[1024];
 	sigset_t set;
 	pid_t pid;