Add support for permission modes in octal in --statoverride files

[jlebon]

This would make writing them less error-prone.

[dbnicholson]

I think that just changing g_ascii_strtod to g_ascii_strtoull with a base of 0 would do the trick as that inteprets a leading 0 as octal. But then the documentation would have to be clear that you need a leading 0 since people often leave that off when using chmod.