Track whether _finish() has been called, and deliver all partial
results then.  Fixes Mantis ticket 0003621.

Implements Mantis ticket 0003549.

These two strings are common "FILE" types.

To replace libcue, the unmaintained and crashy library.

.. rather then append to the end of the previous one

Cuebreakpoints from the cuetools package has three modes of operation,
and the default is to append pregap (INDEX 00) to the end of the
previous track. This is the behavior most compliant to the existing
cue files.

Here is the patch which fixes the issue. I borrowed bits of
implementation from cuebreakpoints. I assumed that the whole audio
file must be covered by head-to-head going tracks, which is how
hardware CD players probably work. In cue_tag I changed rounding from
rounding up to rounding down because the thing in mpd which calculates
actual track duration (and current position) rounds it down, and I
didn't want to see in my playlist values different from whose in a
now-playing progress bar.

I've compared the resultant mpd behaviour with "mplayer -ss MM:SS.MS"
where the time was supplied by cuebreakpoints and noticed that mplayer
started each track a bit earlier then mpd, though this was the same
before the patch.

Like tag_merge(), but can deal with NULL parameters, and frees both
tag objects.

Get duration from track_get_length().

Merge code from cue_tag_file() and cue_tag_string().

It's illegal to pass NULL here.  This should not be ignored silently.

Removed the NULL check.

After we've been hit by Large File Support problems several times in
the past week (which only occur on 32 bit platforms, which I don't
have), this is yet another attempt to fix the issue.

With these methods a tag struct can be created
from the cdtext information in a cue sheet.
The methods depend on a cue parsing library.
Reading from strings (char*) as well as from
a file (FILE*) is supported.