API: libcrmcommon: Add pcmk_section_e.

This enum is used to control which sections will be displayed by tools.
Each item roughly maps to a single formatted output message or a single
--include=/--exclude= option in crm_mon, with two exceptions.

• There is no single "summary" message, but there is a --include=summary that is used to set multiple pcmk_section_ flags at once.
• There is kind of a single "fencing" message, but it's only useful in the XML formatter. Aside from that, there is --include=fencing that sets multiple pcmk_section_ flags at once.