libpromises: limit readstringarrayidx() tokens to CF_MAXVARSIZE

Note: this bug may apply to earlier versions, 3.5 etc.

A very strange case: my system scripts did append mount options to
nfs mounts on /etc/mtab , causing some lines to have >2.5k fields!

Then, new promise:
   => readstringarrayidx("mounts",.., "\s*#[^\n]*", "\s+",..)
of inventory/any.cf:147 borked, because the mount-options column
did have >2k length.

In my case SILENTLY dropping the rest of the token works.