udev: replace unsafe characters on assigning ENV{key}="val" when OPTIONS="string_esca...

udev: replace unsafe characters on assigning ENV{key}="val" when OPTIONS="string_escape=replace" is set

Strictly speaking, this breaks backward compatibility, as previously
`ENV{key}="val"` ignored `string_escape=` option. But, introducing
a new option such as `string_escape=hoge` sounds overkill for me.
The default escape mode is `ESCAPE_UNSET`, so I hope this merely break
existing rules.