sessiond: split event notifier error counter CLI options

With this commit, users can specify the size of event notifier error
counting buffers for each domain independently using the following new
lttng-sessiond options:
  --event-notifier-error-buffer-size-kernel=
  --event-notifier-error-buffer-size-userspace=

The index allocation is now also per-domain meaning that index
allocation in the kernel domain doesn't affect available indices in the
userspace domain and reversely.

Small changes:
- Add manual page description of the new options,
- Rename `struct error_account_entry` to
  `struct ust_error_accounting_entry`.

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I4758d32c00cb432be377fd67eaffa11b193bad74