[Toybox] [PATCH] Revert "Don't strlen() potentially long target string each call to strstart()."

[enh]

This reverts commit 068b12adfb4577b8e732f56e979017cd690baec5.

That commit broke a lot of tests, especially on systems using toybox dd,
where bs= no longer works:

  taimen:/ # dd if=/dev/zero bs=1000 count=1000
  dd: not integer: bs=1000
---
 lib/lib.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)
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