use xsnprintf instead of snprintf.

use xsnprintf instead of snprintf.

snprintf is not available on LynxOS, so I changed the calls to snprintf
to calls to xsnprintf, which should be strictly equivalent.

gdb/gdbserver/ChangeLog:

        * utils.c (xsnprintf): Make non-static.
        * server.h: Add xsnprintf declaration.
        * linux-low.c, nto-low.c, target.c, thread-db.c, tracepoint.c:
        replace calls to snprintf by calls to xsnprintf throughout.