Remove "--enable-cached-man-pages" switch and make rst2man optional when required man pages already exist

Fixes #52
remove doc from rsyslog project itself

in the future, doc stems back to the rsyslog-doc project
use cache copy of rsyslog-doc porject; distribute it
prepare for v8.2.0
add support for specifying the liblogging-stdlog channel spec

new global parameter "stdlog.channelspec"
Merge branch 'v8-stable'