[PATCH v2] clone.2: note EINVAL when exit_signal + bad flags

[Jack Pearson <jack@pearson.onl>]

Document that Linux will report EINVAL when exit_signal is specified and
either CLONE_THREAD or CLONE_PARENT is specified.

From clone3_args_valid in Linux:
```
	if ((kargs->flags & (CLONE_THREAD | CLONE_PARENT)) &&
	    kargs->exit_signal)
		return false;
```

I have verified that this happens on my kernel with a small program:

```
#include <stdio.h>
#include <linux/sched.h>
#include <signal.h>
#include <sys/syscall.h>
#include <unistd.h>

int main() {
	struct clone_args ca = {
		.flags = CLONE_THREAD | CLONE_SIGHAND | CLONE_VM,
		.exit_signal = SIGCHLD, // comment me out to fix error
		.set_tid_size = 0,
	};
	syscall(SYS_clone3, &ca, sizeof(struct clone_args));
	perror("");
}
```

And I have verified that this doesn't happen with normal `clone` through
the glibc helper:

```
#define _GNU_SOURCE

#include <sched.h>
#include <signal.h>
#include <stdio.h>
#include <sys/mman.h>

int do_nothing(void *_) { return 0; }

int main() {
        void *map = mmap(NULL, 0x10000, PROT_READ | PROT_WRITE,
	                 MAP_ANONYMOUS | MAP_PRIVATE, 0, 0);
	void *stack_top = map + 0x10000 - 1;
	clone(do_nothing, stack_top,
	      CLONE_THREAD | CLONE_VM | CLONE_SIGHAND | SIGCHLD, NULL);
	perror("");
}
```

Signed-off-by: Jack Pearson <jack@pearson.onl>
---
 man2/clone.2 | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/man2/clone.2 b/man2/clone.2
index f04d713d1..3829d04c3 100644
--- a/man2/clone.2
+++ b/man2/clone.2
@@ -1435,6 +1435,16 @@ One of the PIDs specified in
 .I set_tid
 was an invalid.
 .TP
+.BR EINVAL " (" clone3 "() only)"
+.\" commit 7f192e3cd316ba58c88dfa26796cf77789dd9872
+.B CLONE_THREAD
+or
+.B CLONE_PARENT
+was specified in the
+.I flags
+mask, but a signal was specified in
+.I exit_signal.
+.TP
 .BR EINVAL " (AArch64 only, Linux 4.6 and earlier)"
 .I stack
 was not aligned to a 128-bit boundary.
-- 
2.35.1