libsensors: No longer depend on libsysfs.
Instead, access sysfs directly, using 3 embedded helper
functions. My motivations for doing this are:
* As far as I know, libsysfs is no longer maintained.
* libsysfs does much more than we need. For example, when asking for a
device attribute list, libsysfs will read the contents and permissions
of all attributes. Not only does this waste CPU cycles per se, but in
the case of hwmon driver it also triggers register reads, which can be
slow for SMBus chips.
* libsysfs enforces the difference between devices and class devices,
while future changes will be easier if we can handle both types alike.