slirp: Replace u_int8_t, u_int16_t, u_int32_t, u_int64_t by standard int types

There is no need to have a second set of integral types.
Replace them by the standard types from stdint.h.

Signed-off-by: Stefan Weil <weil@mail.berlios.de>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>