Bugfix: Last character of file after conversion from code point is not printed by...

Bugfix: Last character of file after conversion from code point is not printed by utf8 program.

The algorithm doesn't print the character until it knows when the character is complete.
There are no checks for when end of file is reached.
This results in the last character not being printed, even if the code is complete.

Be sure to return the status rather than always returning F_none under certain circumstances in utf8_detect_codepoint().
Update documentation about return value in utf8_detect_codepoint().
Initialize the character.used to 0 rather than 4 (because it has no data!).
For better practice, compare using >= rather than ==.
Remove unnecessary i = 0 assignment.