optimize get_current_dir_name to reduce stack bloat

our getcwd already (as an extension) supports allocation of a buffer
when the buffer argument is a null pointer, so there's no need to
duplicate the allocation logic in this wrapper function. duplicating
it is actually harmful in that it doubles the stack usage from
PATH_MAX to 2*PATH_MAX.

src/misc/get_current_dir_name.c

@@ -7,11 +7,9 @@
 
 char *get_current_dir_name(void) {
 	struct stat a, b;
-	char buf[PATH_MAX];
 	char *res = getenv("PWD");
 	if (res && *res && !stat(res, &a) && !stat(".", &b)
 	    && (a.st_dev == b.st_dev) && (a.st_ino == b.st_ino))
 		return strdup(res);
-	if(!getcwd(buf, sizeof(buf))) return NULL;
-	return strdup(buf);
+	return getcwd(0, 0);
 }