runparts: fix arg handling and skip backup files

For some specialized forms of runparts, e.g., start-stop scripts in SysV
init replacements, there is a need to append an argument.  Typically
this is 'start' or 'stop'.

This functionality has been broken for quite some time, meaning we
should consider it being unused by current users of Finit.

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

src/.gitignore

@@ -5,6 +5,7 @@ initctl
 keventd
 logit
 reboot
+runparts
 sulogin
 watchdogd
 Makefile