Reapply "Explicitly validate popen mode"

To avoid behavior differences due to libc. This time with the
check only for the non-win32 case, as Windows support additional
modifiers here (t/b).

Author: nikic

ext/standard/file.c

@@ -930,8 +930,16 @@ PHP_FUNCTION(popen)
 		char *z = memchr(posix_mode, 'b', mode_len);
 		if (z) {
 			memmove(z, z + 1, mode_len - (z - posix_mode));
+			mode_len--;
 		}
 	}
+
+	/* Musl only partially validates the mode. Manually check it to ensure consistent behavior. */
+	if (mode_len != 1 || (*posix_mode != 'r' && *posix_mode != 'w')) {
+		php_error_docref2(NULL, command, posix_mode, E_WARNING, "Invalid mode");
+		efree(posix_mode);
+		RETURN_FALSE;
+	}
 #endif
 
 	fp = VCWD_POPEN(command, posix_mode);