dehacked: Eliminate some more uses of strncpy().

Use M_StringCopy() instead of strncpy() in the Dehacked code.
Eliminate some uses of sprintf() from common code.

As part of this, add DIR_SEPARATOR_S as a string version of the
DIR_SEPARATOR macro. Change M_TempFile() to return a string allocated
on the C heap rather than the zone heap.

This is a first step towards fixing #371.
music: Free string using free() rather than Z_Free().

Mistake introduced in previous commit.
doom: Eliminate use of sprintf().

Use snprintf() or other functions in place of sprintf(). This is part
of fixing #371.