relibc: implement getprogname() in stdlib

BSD extension used by diffutils, coreutils, and other GNU software.
Returns the basename of program_invocation_name.

src/header/stdlib/mod.rs

@@ -457,6 +457,23 @@ pub unsafe extern "C" fn getenv(name: *const c_char) -> *mut c_char {
         .unwrap_or(ptr::null_mut())
 }
 
+#[unsafe(no_mangle)]
+pub unsafe extern "C" fn getprogname() -> *const c_char {
+    let full = crate::platform::program_invocation_name;
+    if full.is_null() {
+        return c"".as_ptr() as *const c_char;
+    }
+    let mut base = full;
+    let mut i = full;
+    while unsafe { *i } != 0 {
+        if unsafe { *i } == b'/' as c_char {
+            base = unsafe { i.add(1) };
+        }
+        i = unsafe { i.add(1) };
+    }
+    base
+}
+
 /// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/getsubopt.html>.
 #[unsafe(no_mangle)]
 pub unsafe extern "C" fn getsubopt(