tooltalk: Fix bad assumptions about sizeof(uid_t)

tooltalk: Fix bad assumptions about sizeof(uid_t)

In part of the tooltalk rpc code (mp_message.c), it was assumed that on
the majority of platforms, sizeof(uid_t)=sizeof(gid_t)=sizeof(long).  On
Linux-x64, uid_t is an unsigned int, which makes the code fail: all
tooltalk messages fail to send with an RPC_CANTENCODEARGS at the
rpc-level, and TT_INTERNAL_ERR for the actual program.  We instead
change the code to explicitly examine sizeof(uid_t) to see whether it is
int or long sized. This allows tooltalk-dependent functinoality
like logout and multiple calls to dtfile to work.