add method to get/set charge voltage

[arne.anka]

frameworkd should offer a set of methods to get/set the voltage currently available when charging from usb.

i got an external battery "just mobile gum pro" which apparently is not recognized and thus does not charge the fr correctly. setting usb_curlim requires root access and is nevertheless a task, fso could very well handle.

additionally, the path below /sys/ differes between different kernel versions -- a unified call from fso would solve the confusion in userland.

since the value of the current limit is relevant for userland apps and every setter should have a getter anyway, besides setting, getting the value, too, is desirable.