This patch creates two utility functions:

  int open_cloexec(pathname, flags)
  int create_cloexec(pathname, flags, mode)

These return a file descriptor with the CLOEXEC flag set, to ensure
that the descriptor is not inherited across a fork/exec operation.

The sockets returned by:

   _x_io_tcp_connect_ipv4()
   _x_io_tcp_connect()

now also have their CLOEXEC flag set.

This requires that many other files include config.h themselves.
Also convert <config.h> to "config.h".

Add warning flags to the DEBUG_CFLAGS too.

Using asprintf() instead of malloc() + sprintf() reduces the lines of
code in xine-lib (moving the allocation to the C library or asprintf
replacement), makes it safer to access the string and can also improve
performance whenever the value returned by a function was used as
parameter, as before it had to run the function twice in almost every
case (once for strlen(), once for sprintf()).