[PATCH] ash: improve speed of variable pattern substitution

[Ron Yorston]

I see that Denys has applied a smaller and tidier alternative to
my attempt at a patch.  I note:

- The speed up is still only applied if a non-default build option
  is selected so there's no change in the size of a default build.

- The slower search is used if any special characters are present in
  the pattern even if they're escaped.  Thus ${x//\*/|}, for example,
  will continue to be slow.

Ron