Restructure existing man pages according to JEP 299

In preparation for JDK-8178317, the existing man page troff sources will be moved to their corresponding modules, according to the layout determined by JEP 299. During the docs build, they will be copied to the man directory in the docs image, as specified by JEP 299. No markdown conversion will be done for now, though.

Note that this will only apply to OpenJDK, not the Oracle JDK, which do not ship the man pages from the OpenJDK repository.