Here we replace the various fgets() with a mmap() call for the config
file reading, due to the reported performance problems with klibc.
Thanks to Patrick's testing, it makes a very small, close to nothing
speed gain for libc users, but a 6 times speed increase for klibc users
with a 1000 line config file.
I've created a udev_lib.[hc] for this and also moved all the generic
stuff from udev.h in there and uninlined the functions.
