maint: prep for global quoting changes: handle irregular cases manually

maint: prep for global quoting changes: handle irregular cases manually

* src/chroot.c (usage): Change ``...'' to '...', and describe the
default more accurately (also adding quotes): s,/bin/sh,'/bin/sh -i',
* src/join.c (usage): Change ` ...'' ' to "...''".
* src/fmt.c (isopen): Change `' to '` in list of bytes, so that
a subsequent change can safely perform the `...' to '...' conversion.
* src/truncate.c (main): Tweak quoting in comments to use '...',
not `...`, for consistency with the rest of comments in coreutils.