provide emulation of fcntl F_DUPFD_CLOEXEC on old kernels

I'm not entirely happy with the amount of ugliness here, but since
F_DUPFD_CLOEXEC is used elsewhere in code that's expected to work on
old kernels (popen), it seems necessary. reportedly even some modern
kernels went back and broke F_DUPFD_CLOEXEC (making it behave like
plain F_DUPFD), so it might be necessary to add some additional fixup
code later to deal with that issue too.

src/fcntl/fcntl.c

@@ -22,5 +22,21 @@ int fcntl(int fd, int cmd, ...)
 		if (ret) return __syscall_ret(ret);
 		return ex.type == F_OWNER_PGRP ? -ex.pid : ex.pid;
 	}
+	if (cmd == F_DUPFD_CLOEXEC) {
+		int ret = __syscall(SYS_fcntl, fd, F_DUPFD_CLOEXEC, arg);
+		if (ret != -EINVAL) {
+			if (ret >= 0)
+				__syscall(SYS_fcntl, ret, F_SETFD, FD_CLOEXEC);
+			return __syscall_ret(ret);
+		}
+		ret = __syscall(SYS_fcntl, fd, F_DUPFD_CLOEXEC, 0);
+		if (ret != -EINVAL) {
+			if (ret >= 0) __syscall(SYS_close, ret);
+			return __syscall_ret(-EINVAL);
+		}
+		ret = __syscall(SYS_fcntl, fd, F_DUPFD, arg);
+		if (ret >= 0) __syscall(SYS_fcntl, ret, F_SETFD, FD_CLOEXEC);
+		return __syscall_ret(ret);
+	}
 	return syscall(SYS_fcntl, fd, cmd, arg);
 }