Don't use 'bool' as a struct member name in help_config.c.

 Don't use 'bool' as a struct member name in help_config.c.
 
 Doing so doesn't work if bool is a macro rather than a typedef.
 
 Although c.h spends some effort to support configurations where bool is
 a preexisting macro, help_config.c has existed this way since
 2003 (b700a6), and there have not been any reports of
 problems. Backpatch anyway since this is as riskless as it gets.
 
 Discussion: 20150812084351.GD8470@awork2.anarazel.de
 Backpatch: 9.0-master