gdb: convert 'set debug lin-lwp' to a boolean command

gdb: convert 'set debug lin-lwp' to a boolean command

Convert the 'set debug lin-lwp' command to a boolean.  Adds a new
LINUX_NAT_SCOPED_DEBUG_ENTER_EXIT macro, and makes use of it in one
place (linux_nat_target::stop).

The manual entry for 'set debug lin-lwp' is already vague about
exactly what arguments this command takes, and the description talks
about turning debug on and off, so I don't think there's any updates
required there.

I have updated the doc strings shown when the users enters 'help show
debug lin-lwp' or 'help show debug lin-lwp'.  The old title lines used
to talk about the 'GNU/Linux lwp module', but this debug flag is now
used for any native linux target debug, so we now talk about
'GNU/Linux native target'.  The body string for this setting has been
changed from 'Enables printf debugging output.' to 'When on, print
debug messages relating to the GNU/Linux native target.', the old
value looks like a cut&paste error to me.