All debug output needs to go via debug functions to ensure it writes to the
correct output stream.
gdb/ChangeLog:
* nat/linux-waitpid.c (linux_debug): Call debug_vprintf.
gdb/gdbserver/ChangeLog:
* ax.c (ax_vdebug): Call debug_printf.
* debug.c (debug_write): New function.
* debug.h (debug_write): New declaration.
* linux-low.c (sigchld_handler): Call debug_write.
