Fix: Replace sighandler_t with `sig_t for consistency across platforms

    - Result: Tests pass
    - Check Apple type at local
        ```bash
        grep -r "sig_t" /usr/include /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include 2>/dev/null
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/kern/kcdata.h:	uint8_t   ceri_page_codesig_tainted;
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/sys/signal.h:typedef void (*sig_t)(int);     /* type of signal function */
        ```
        > Aarch MacOS 26.3.1
    - Check Other OS from the Internet

    Source:

        - (Freebsd) https://github.com/freebsd/freebsd-src/blob/016570c4463d5908953355ee1cf9a385ad9601b4/sys/sys/signal.h
        - (Dragonfly) https://github.com/DragonFlyBSD/DragonFlyBSD/blob/51591aff67978c86de99839e31d8c8fcc54bc316/sys/sys/signal.h
        - https://www.gnu.org/software/gnulib/manual/html_node/signal_002eh.html
            - The type sighandler_t (a GNU extension) is not defined on most non-glibc platforms: macOS 11.1, FreeBSD 14.0, NetBSD 10.0, OpenBSD 7.5, AIX 5.1, HP-UX 11, Solaris 11.4, Cygwin, mingw, MSVC 14.

Author: chengr4

src/unix/bsd/apple/mod.rs

@@ -327,7 +327,7 @@ s! {
 
     pub struct sigaction {
         // FIXME(union): this field is actually a union
-        pub sa_sigaction: crate::sighandler_t,
+        pub sa_sigaction: crate::sig_t,
         pub sa_mask: sigset_t,
         pub sa_flags: c_int,
     }

src/unix/bsd/freebsdlike/mod.rs

@@ -164,7 +164,7 @@ s! {
     }
 
     pub struct sigaction {
-        pub sa_sigaction: crate::sighandler_t,
+        pub sa_sigaction: crate::sig_t,
         pub sa_flags: c_int,
         pub sa_mask: sigset_t,
     }

src/unix/bsd/netbsdlike/mod.rs

@@ -27,7 +27,7 @@ s! {
     }
 
     pub struct sigaction {
-        pub sa_sigaction: crate::sighandler_t,
+        pub sa_sigaction: crate::sig_t,
         pub sa_mask: crate::sigset_t,
         pub sa_flags: c_int,
     }

src/unix/mod.rs

@@ -18,6 +18,18 @@ pub type pid_t = i32;
 pub type in_addr_t = u32;
 pub type in_port_t = u16;
 pub type sighandler_t = size_t;
+#[cfg(any(
+    target_os = "macos",
+    target_os = "ios",
+    target_os = "tvos",
+    target_os = "watchos",
+    target_os = "visionos",
+    target_os = "freebsd",
+    target_os = "dragonfly",
+    target_os = "openbsd",
+    target_os = "netbsd"
+))]
+pub type sig_t = sighandler_t;
 pub type cc_t = c_uchar;
 
 cfg_if! {