Fix POSIX signal(3) binding's type signature; remove bsd_signal (#24400)

POSIX signal has an identical definition to ISO C signal:
https://pubs.opengroup.org/onlinepubs/9799919799/functions/signal.html

```c
void (*signal(int sig, void (*func)(int)))(int);

/* more readably restated by glibc as */
typedef void (*sighandler_t)(int);

sighandler_t signal(int signum, sighandler_t handler);
```

However, std/posix had omitted the function's return value; this fixes
that.

To prevent breaking every single line of code ever that touched this
binding (including mine...), I've also made it discardable.

Additionally, I have noticed that bsd_signal's type signature is wrong -
it should have been identical to signal. But bsd_signal was already
removed in POSIX 2008, and sigaction is the recommended, portable POSIX
signal interface. So I just deleted the bsd_signal binding.

Co-authored-by: ringabout <[email protected]>

Author: bptato

lib/posix/posix.nim

@@ -783,8 +783,6 @@ const
 proc getrusage*(who: cint, rusage: ptr Rusage): cint
   {.importc, header: "<sys/resource.h>", discardable.}
 
-proc bsd_signal*(a1: cint, a2: proc (x: pointer) {.noconv.}) {.
-  importc, header: "<signal.h>".}
 proc kill*(a1: Pid, a2: cint): cint {.importc, header: "<signal.h>", sideEffect.}
 proc killpg*(a1: Pid, a2: cint): cint {.importc, header: "<signal.h>", sideEffect.}
 proc pthread_kill*(a1: Pthread, a2: cint): cint {.importc, header: "<signal.h>".}
@@ -806,8 +804,8 @@ proc sighold*(a1: cint): cint {.importc, header: "<signal.h>".}
 proc sigignore*(a1: cint): cint {.importc, header: "<signal.h>".}
 proc siginterrupt*(a1, a2: cint): cint {.importc, header: "<signal.h>".}
 proc sigismember*(a1: var Sigset, a2: cint): cint {.importc, header: "<signal.h>".}
-proc signal*(a1: cint, a2: Sighandler) {.
-  importc, header: "<signal.h>".}
+proc signal*(a1: cint, a2: Sighandler): Sighandler {.
+  importc, discardable, header: "<signal.h>".}
 proc sigpause*(a1: cint): cint {.importc, header: "<signal.h>".}
 proc sigpending*(a1: var Sigset): cint {.importc, header: "<signal.h>".}
 proc sigprocmask*(a1: cint, a2, a3: var Sigset): cint {.