maint: fix message translation glitches

maint: fix message translation glitches

Problem reported by Sebastian Rasmussen in: http://bugs.gnu.org/18054
* gl/lib/randread.c (randread_error): Don't put multiple string
literals inside _(...), as xgettext doesn't support that.
* src/chroot.c (main): In diagnostics, don't bother to distinguish
between setting the number of supplemental group IDs to a zero or
to a nonzero value, as the underlying system call is the same
either way.  This also makes the string easier to translate correctly.