maint: port to GCC 8

maint: port to GCC 8

* src/chown-core.h (chopt_free, gid_to_name, uid_to_name):
No longer const.
* src/make-prime-list.c (xalloc): Add malloc attribute.
* src/who.c (make_id_equals_comment): Work around GCC bug 85602
by using mempcpy rather than strncat.  Although the old code
was correct, strncat raises so many hackles that it’s not
worth maintaining its use here.