Fix vacuumdb to pass buffer-usage-limit with analyze-only mode

Fix vacuumdb to pass buffer-usage-limit with analyze-only mode

ae78cae3b added the --buffer-usage-limit to vacuumdb to allow it to
include the BUFFER_USAGE_LIMIT option in the VACUUM command.
Unfortunately, that commit forgot to adjust the code so the option was
added to the ANALYZE command when the -Z command line argument was
specified.

There were no issues with the -z command as that option just adds
ANALYZE to the VACUUM command.

In passing adjust the code to escape the --buffer-usage-limit option
before passing it to the server.  It seems nothing beyond a confusing
error message could become this lack of escaping as VACUUM cannot be
specified in a multi-command string.

Reported-by: Ryoga Yoshida
Author: Ryoga Yoshida, David Rowley
Discussion: https://postgr.es/m/08930c0b541700a5264e5fbf3a685f5a%40oss.nttdata.com
Backpatch-through: 16, where ae78cae3b was introduced.