Fixes #3351: site-install Drupal 7 with PHP 7.2

[greg-1-anderson]

Always use 'fwrite' instead of 'print' in 'drush_print' to avoid undesired interaction with PHP's handling of the php headers.

[greg-1-anderson]

A couple of tests fail because this change interferes with the way they were collecting output

[ryanaslett]

I'll give this a whirl tomorrow morning

[greg-1-anderson]

The test failures were happening because output emitted via fwrite(STDOUT) is not captured by ob_start() / ob_end(). I adjusted for this by explicitly capturing output in drush_print().

The side-effect here is that if anyone is emitting output via print instead of drush_print, it has the potential to do the wrong thing. It has always been deprecated to call print instead of drush_print, though, so this is not a serious issue.

We have two options:

1. Leave this PR as-is.
2. Conditionalize this change on the php version, so that PHP 7.1 and lower continue to use the current implementation, and PHP 7.2 and onward use the implementation in this PR.

The later strategy would be better for strict maintenance of backwards compatibility; however, since we are only breaking non-conforming code here, I think it is probably okay to just make the change. I'm willing to go either way, though, depending on what folks think.

[weitzman]

I'm fine with the PR as is.