Support include and libs in separate dirs

Support Serval include directory and lib
directory in separate places. Only set them
in the Makefile if they are not set on the
command line.
Use proper type of length variable
keyring_open_with_pins() is gone

Replace it with a call to
keyring_open_instance() which does
almost exactly the same thing!
Merge pull request #5 from hawkinswnaf/release-0.6.5.4

keyring_open_with_pins() is gone (plus Makefile spiff-up) - Still needs additional testing.