Add manpage XSL from git and enhance with literallayout fixes

It ensures that .sp tags will be properly output so they do not appear
in the rendered manpages. Grabed from git's Documentation/callouts.xsl
file. From the git's commit 7ef0435088f41165ece95b6f226d3c15438505a5:

    This is just a random hack to work around problems people seem
    to be seeing in manpage backend of xmlto (it appears we are
    getting ".sp" at the end of line without line break).

Additionally, output an empty line after literallayout blocks (.nf .fi)
so that any text following it will appear in a separate block.