vslc: Plug mmap file descriptor leak

To be consistent with how the file cursor behaves, the close_fd field is
duplicated in the mmap cursor. If a VUT replaces stdin's file descriptor
with a regular file's fd using dup2(2), we don't want to close it just
because we managed to mmap(2) it.

For some reason we don't use the closefd() macro in the VSL cursor code,
potentially to avoid its underlying assertion in libvarnishapi.

On the other hand we do use it in other places:

    $ git grep -l closefd -- lib/libvarnishapi/
    lib/libvarnishapi/daemon.c
    lib/libvarnishapi/vsm.c

So maybe in a subsequent change `(void)close(fd)` statements could turn
into `closefd(&fd)` in vsl_cursor.c to harden those code paths as well.

lib/libvarnishapi/vsl_cursor.c

@@ -409,6 +409,7 @@ struct vslc_mmap {
 	unsigned			magic;
 #define VSLC_MMAP_MAGIC			0x7de15f61
 	int				fd;
+	int				close_fd;
 	char				*b;
 	char				*e;
 	struct VSL_cursor		cursor;
@@ -424,6 +425,8 @@ vslc_mmap_delete(const struct VSL_cursor *cursor)
 	CAST_OBJ_NOTNULL(c, cursor->priv_data, VSLC_MMAP_MAGIC);
 	assert(&c->cursor == cursor);
 	AZ(munmap(c->b, c->e - c->b));
+	if (c->close_fd)
+		(void)close(c->fd);
 	FREE_OBJ(c);
 }
 
@@ -480,7 +483,7 @@ static const struct vslc_tbl vslc_mmap_tbl = {
 };
 
 static struct VSL_cursor *
-vsl_cursor_mmap(struct VSL_data *vsl, int fd)
+vsl_cursor_mmap(struct VSL_data *vsl, int fd, int close_fd)
 {
 	struct vslc_mmap *c;
 	struct stat st[1];
@@ -500,7 +503,8 @@ vsl_cursor_mmap(struct VSL_data *vsl, int fd)
 	ALLOC_OBJ(c, VSLC_MMAP_MAGIC);
 	if (c == NULL) {
 		(void)munmap(p, st->st_size);
-		(void)close(fd);
+		if (close_fd)
+			(void)close(fd);
 		vsl_diag(vsl, "Out of memory");
 		return (NULL);
 	}
@@ -508,6 +512,7 @@ vsl_cursor_mmap(struct VSL_data *vsl, int fd)
 	c->cursor.priv_data = c;
 
 	c->fd = fd;
+	c->close_fd = close_fd;
 	c->b = p;
 	c->e = c->b + st->st_size;
 	c->next.ptr = TRUST_ME(c->b + sizeof VSL_FILE_ID);
@@ -557,7 +562,7 @@ VSL_CursorFile(struct VSL_data *vsl, const char *name, unsigned options)
 		return (NULL);
 	}
 
-	mc = vsl_cursor_mmap(vsl, fd);
+	mc = vsl_cursor_mmap(vsl, fd, close_fd);
 	if (mc == NULL)
 		return (NULL);
 	if (mc != MAP_FAILED)