Integrate changes #18422 and #18423 from maint-5.8.

Integrate changes #18422 and #18423 from maint-5.8.
18422 :
Third Degree found a read of uninit memory (after setting
grent_size with sysconf(), pwent_size was read).  Also make
the ERANGE/INVARG more consistent.
18423 :
spent_size was still off.  (As far as I can tell
_SC_GETPW_R_SIZE_MAX is supposed to be used also
for spents, not just pwents.)
p4raw-link: @18422 on //depot/maint-5.8/perl: c45c44aa17d6498a967957363477eb572b803726

p4raw-id: //depot/perl@18498
p4raw-integrated: from //depot/maint-5.8/perl@18497 'copy in' reentr.h
	(@17645..) reentr.c reentr.pl (@18422..)