Use bool for boolean in xsmfns.c

* xsmfns.c, xterm.h (x_session_have_connection):
* xsmfns.c (doing_interact, smc_interact_CB, Fhandle_save_session):
Use bool for boolean.
(x_session_initialize, Fhandle_save_session):
Prefer NILP (x) to EQ (x, Qnil).