Error messages should be sentence fragments, and therefore:

1. Should not begin with a capital letter,
2. Should not conclude with punctuation, and
3. Should not end a sentence and begin a new one

The first time may be due to memory fragmentation or just bad luck on a
32-bit system. When we hit the mmap error for the first time, free up
the unused windows and try again.

Without this change, compiling with gcc and pedantic generates warning:
ISO C does not allow extra ‘;’ outside of a function.

Set a message when we fail to lock.

Also make the put function void, since it's called from free, which
cannot report errors. The only errors we can experience here are
internal state corruption, so we assert that we are trying to put a
pack which we have previously got.

If we fail to insert the packfile in the map, make sure to free it.

This makes the free function only attempt to remove its mwindows from
the global list if we have opened the packfile to avoid accessing the
list unlocked.

Opening the same repository multiple times will currently open the same
file multiple times, as well as map the same region of the file multiple
times. This is not necessary, as the packfile data is immutable.

Instead of opening and closing packfiles directly, introduce an
indirection and allocate packfiles globally. This does mean locking on
each packfile open, but we already use this lock for the global mwindow
list so it doesn't introduce a new contention point.