make logout_req::reason an unsigned char to fix warning setting bit 7

[emaste]

Summary

Based on context it appears reason should be an unsigned 8-bit field.

/tank/emaste/src/freebsd-clang-import/sbin/iscontrol/fsm.c:612:25: error: implicit conversion from 'int' to 'char' changes value from 128 to -128 [-Werror,-Wconstant-conversion]
     p->reason = BIT(7) | 0;
               ~ ~~~~~~~^~~
1 error generated.
*** [fsm.o] Error code 1 (ignored)

Oddly I can't find any user of logout_req(_t)? other than the one in sbin/iscontrol/fsm.c

[dim]

It looks good to me, but I'm not sure if there are other consumers of this header, which rely on the fields? Then again, u_char is clearly the right type here.