Split usounds.c and usystem.c from xboard.c

Split usounds.c and usystem.c from xboard.c

X-independent code was moved to separate files. The nef file usound.c
contains the sound handling, which in XBoard is trivial, because it uses
an external player, but could be replaced by a complex one on platforms
that have to handle the sound themselves.
  The usystem.c file contains all non-X Unix-like stuff, such as fork,
pipe, system, getenv. I also contains some color parsing for the xterm
colors. This stuff could be useful on platforms that support Unix-like
system calls.