boolean/auto-boolean commands, make "o" ambiguous

boolean/auto-boolean commands, make "o" ambiguous

We currently accept "o" with boolean/auto-boolean commands, taking it
to mean "on".  But "o" is ambiguous, between "on" and "off".  I can't
imagine why assuming the user wanted to type "on" is a good idea, it
might have been a typo.

This commit makes gdb error out.  We now get:

 (gdb) maint test-settings set boolean o
 "on" or "off" expected.

 (gdb) maint test-settings set auto-boolean o
 "on", "off" or "auto" expected.

gdb/ChangeLog:
2019-06-13  Pedro Alves  <palves@redhat.com>

	* cli/cli-setshow.c (parse_auto_binary_operation)
	(parse_cli_boolean_value): Don't allow "o".

gdb/testsuite/ChangeLog:
2019-06-13  Pedro Alves  <palves@redhat.com>

	* gdb.base/settings.exp (test-boolean, test-auto-boolean): Check
	that "o" is ambiguous.