Scaffolding for YAML-formatted cache manager reports (#1784)

Scaffolding for YAML-formatted cache manager reports (#1784)

The only YAML-compliant report currently using new code is mgr:pconn.

Non-YAML non-aggregated reports in non-SMP instances are now framed
using "by kid0 {...}" wrappers (for `squid -N`) or "by kid1 {...}"
wrappers (for instances having one worker process and no rock diskers).
This change makes YAML and non-YAML report framing more similar, but may
affect existing report parsing automation.

Also fixed Content-Type value computation for SMP reports: They were
missing ";charset=utf-8" suffix. Broken since 2014 commit 8088f8d0,
possibly due to code duplication created by 2010 commit 8822ebee.

Moved FunActionCreator and ClassActionCreator classes into
src/mgr/Registration.cc because no other code needs to know about them.
This is especially valuable for function-based actions because they do
not need to know about Mgr::FunAction (or even Mgr::Action).