gdb/remote: Use true/false instead of 1/0

gdb/remote: Use true/false instead of 1/0

The remote_state::starting_up member variable is already of type bool,
but in some places we still write to it using 1 and 0.  This commit
just updates things to use true and false.

There should be no user visible change after this commit.

gdb/ChangeLog:

	* remote.c (remote_target::start_remote): Set 'starting_up' using
	boolean values instead of integers.