[zlib_bench] improve program options handling

The program options handling has grown a little weedy. Add get_options
helper. The wrapper (gzip|zlib|raw) is a required arg, not an optional
arg, so remove --wrapper. Limit the --compression level to [1-9] since
0 causes compression failure in practice. Use Z_DEFAULT_COMPRESSION if
--compression is not given (zlib@6). Make the compression_level static
global since it is a program-level constant. Update usage helper.

Bug: 798943
Change-Id: I8e19bd029c1163a00a1b6a2a252c1a4e5da1a633
Reviewed-on: https://chromium-review.googlesource.com/c/1460242


Reviewed-by: Chris Blume <cblume@chromium.org>
Commit-Queue: Noel Gordon <noel@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#631624}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 74c7f46559047433d694f20cf574815bfa4cb8a6