Re-fix parsing of legacy url_rewrite_program responses (#2446)

Re-fix parsing of legacy url_rewrite_program responses (#2446)

Legacy helper responses start with a URL instead of `OK rewrite_url=...`
and such. 2016 commit ddc77a2e introduced two bugs when handling legacy
responses:

* Response parsing code triggered MemBuf assertions when 0-terminating
  the parsing buffer for certain URLs. The bug affected legacy helper
  responses with and without space characters.

* Squid code attempted to accept/use helper-returned URLs with embedded
  space character(s), despite a WARNING implying that the post-space
  characters are not going to become a part of the new URL.

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This change resurrects recent commit bb854bb9 that was accidentally
reverted by commit ec328cf16 during pull request merging by Anubis.