Initialize child signal handling before posix_spawn too.

Problem reported by Tino Calancha (Bug#58960).
* src/callproc.c (call_process): Initialize SIGCHLD handling
before possibly creating a child with emacs_span.  This need not
be in the critical section that calls emacs_spawn, so do it
outside the critical section.
* src/process.c (child_signal_init): Now extern.

src/callproc.c

@@ -648,6 +648,7 @@ call_process (ptrdiff_t nargs, Lisp_Object *args, int filefd,
 
 #ifndef MSDOS
 
+  child_signal_init ();
   block_input ();
   block_child_signal (&oldset);
 

src/lisp.h

@@ -4915,6 +4915,7 @@ extern bool running_asynch_code;
 
 /* Defined in process.c.  */
 struct Lisp_Process;
+extern void child_signal_init (void);
 extern void kill_buffer_processes (Lisp_Object);
 extern int wait_reading_process_output (intmax_t, int, int, bool, Lisp_Object,
 					struct Lisp_Process *, int);

src/process.c

@@ -292,7 +292,6 @@ static int child_signal_read_fd = -1;
    descriptor to notify `wait_reading_process_output' of process
    status changes.  */
 static int child_signal_write_fd = -1;
-static void child_signal_init (void);
 #ifndef WINDOWSNT
 static void child_signal_read (int, void *);
 #endif
@@ -7323,7 +7322,7 @@ process has been transmitted to the serial port.  */)
 
 /* Set up `child_signal_read_fd' and `child_signal_write_fd'.  */
 
-static void
+void
 child_signal_init (void)
 {
   /* Either both are initialized, or both are uninitialized.  */