Optimize raw file copy action to coalesce into few copies.

Boost uses a bunch of individual file copy targets to install the headers files. This is rather slow since it ends up in thousands of copy command processes. Either adjust existing copy actions, and create new actions, for copying files in batches instead.

Start by tracing B2's raw file copy actions and the header installation targets described in the issue. Determine where individual copy targets are created, then verify that installation uses batched copies and substantially fewer copy command processes.

Written by the indexing model from the issue text.