DMG: properly re-enable plain checks, for OpenCL too.
Try to speedup cmp_all() by use of a bitmap.  This only makes sense when
cmp_all() is called multiple times per crypt_all()'s actual processing -
that is, when cracking two or more C/R pairs at once.  There's a check
for that in the code.  Moreover, use of the bitmap is automatically
disabled when the C/R pair count reduces to 1.