Change README to reflect actual license state

The README stated the file to use a fixed license version, but all source
files included the "or later" statement. So sync the README to the actual
licenses.

Signed-off-by: Michael Heimpold <mhei@heimpold.de>
Drop extern keyword: this is non-sense for functions

Signed-off-by: Michael Heimpold <mhei@heimpold.de>
Whitespace fix

Signed-off-by: Michael Heimpold <mhei@heimpold.de>
Rename field fd to fd_value

Signed-off-by: Michael Heimpold <mhei@heimpold.de>
Add some documentation to the higher level API and extend the GPIO context structure

Signed-off-by: Michael Heimpold <mhei@heimpold.de>