ext/standard/exec.c: combine conditions, update docs

While `php_escape_shell_cmd()` did indeed `emalloc` a string that needed to be
freed by the caller in the original implementation that was put in GitHub
(see commit 257de2b) a few months ago the
return type was changed to use `zend_string`, see php#14353.

Closes phpGH-16313

Author: DanielEScherzer

ext/standard/exec.c

@@ -118,11 +118,7 @@ PHPAPI int php_exec(int type, const char *cmd, zval *array, zval *return_value)
 	php_stream *stream;
 	size_t buflen, bufl = 0;
 #if PHP_SIGCHILD
-	void (*sig_handler)() = NULL;
-#endif
-
-#if PHP_SIGCHILD
-	sig_handler = signal (SIGCHLD, SIG_DFL);
+	void (*sig_handler)() = signal(SIGCHLD, SIG_DFL);
 #endif
 
 #ifdef PHP_WIN32
@@ -272,8 +268,7 @@ PHP_FUNCTION(passthru)
    Escape all chars that could possibly be used to
    break out of a shell command
 
-   This function emalloc's a string and returns the pointer.
-   Remember to efree it when done with it.
+   This function returns an owned zend_string, remember to release it when done.
 
    *NOT* safe for binary strings
 */