Revert "[counters] Simplify things a bit by removing unsigned counter types."

This reverts commit 2fa83d9a8cef9d5a5c050e40bc2f757dba445997.
[mono-counters] Bring MONO_COUNTER_TIME_INTERVAL back.
[counters] Install mono-counters.h since it's part of the public API.
[counters] Document mono_counters_foreach.
[counters] Restore the previous behavior of mono_counters_dump, add variance filtering and reorg the bitfield.

The previous behavior when using mono_counters_dump was to pass -1 as the section and get all
counters dumped. This commit restore this behavior.

In addition to that, we want to enable callers to filter by variance as non monotonic counters are not useful
to be dumped at shutdown. To do so you just OR the variance with the sections you want. So what the runtime wants is:

	mono_counters_dump (MONO_COUNTER_SECTION_MASK | MONO_COUNTER_MONOTONIC, stdout);

Dump all counters that are monotonic. Perfect.

Finally, to make the above neat, a reorg of the type bitfield was needed. Variance is now using a bit per kind.
Variance and unit where shrunk to 4 bits each, to open space for more sections.
[counters] Only dump monotonic counters as variable ones are meaningless during shutdown.
Drop Events and Items units in favor of Count as they represent pretty much the same thing.