Subzero: Allow overriding command-line args from the browser.

In the browser build only, allows arguments to be explicitly passed to pnacl-sz, in two ways:

1. The SZARGFILE envvar contains the name of a file with arguments, one per line.  For each line, initial whitespace is ignored, and lines starting with the '#' comment character are also ignored.

2. The SZARGLIST envvar contains all the arguments, separated by the '|' character.

Chrome needs to be started with special options to allow the envvars to be passed through, and also to allow access to the local file system.

In addition, specifying "-log=/dev/stderr" or "-o /dev/stderr" gets mapped to std::cerr, in the same way "-" gets mapped to std::cout.

BUG= https://bugs.chromium.org/p/nativeclient/issues/detail?id=4370
R=jpp@chromium.org

Review URL: https://codereview.chromium.org/1903553004 .