plist_read_from_file() is a convenience function that will open a
given file, checks its size, allocates a buffer large enough to
hold the full contents, and reads from file to fill the buffer.
Then, it calls plist_from_memory() to convert the data to plist
format.
A (breaking) change had to be made so that plist_from_memory() will
also return the parsed format in its 4th argument (if non-NULL).

This makes the code more readable. Obviously all the code that uses it
is also updated.

This properly supports getting and setting signed or unsigned integer values.
Also, a new helper function plist_int_val_is_negative() was added to determine if
a given #PLIST_INT node has a negative value or not.

The old type PLIST_UINT is defined as a macro with the value of PLIST_INT for
backwards compatibility.

This commit also adds int vs. uint support to the C++ interface, and the python
bindings in a hopefully useful way.