Add --clear-chicken-home command line option

Contrary to what the commit message of 5b13e99 says, setting
CHICKEN_PREFIX is not enough to prevent eggs like setup-helper to
install Scheme files in (chicken-home).  5b13e99 can't actually
set CHICKEN_PREFIX to tmp-repo-dir (and it didn't), because the
CHICKEN tools are not there.  5b13e99 sets CHICKEN_PREFIX to
chicken-installation-prefix, complementary to
what "chicken-install -p" does, but that's still ineffective to
prevent the installation of Scheme files in (chicken-home),
since (chicken-home) will still point to
<chicken-installation-prefix>/share/chicken.

Preventing the installation of files to (chicken-home) can't be
easily implemented.  So it's easier to let eggs install files
in (chicken-home), then remove them.

Since <chicken-installation-prefix> is out of salmonella's
tmp-repo-dir, it can't just remove files from there.  This commit
introduces a new command line option, --clear-chicken-home, to
specify that salmonella can actually remove files from
<chicken-installation-prefix>/share/chicken.  This option must be
used with care.
Whitespace cleanup