player: don't sort a NULL list

This can occur if the directory does not have any files in it which
causes files to never be non-NULL for qsort.
player: simplify chapter display name code

The display name is always recomputed, so we can always toss the input
name.
tv: initialize frequencies to 0
posix: use STD*_FILENO constants

Rather than "magic" numbers, use meaningful constant names provided by
unistd.h.