Added support for writing man files into a specific output

Added support for writing man files into a specific output
directory and into appropriate subdirectories within that output
directory. Controlled through the man.base.dir parameter (similar
to the base.dir support in the HTML stylesheet) and the
man.subdirs.enabled parameter, which automatically determines the
name of an appropriate subdir (for example, man/man7, man/man1,
etc.) based on the section number/manvolnum of the source Refentry.

Closes #1255036 and #1170317. Thanks to Denis Bradford for the
original feature request, and to Costin Stroie for submitting a
patch that was very helpful in implementing the support.