MAC/do: Output errors when parsing rules

[olce]

Summary

This revision is part of a series. Click on the Stack tab below to see the context.
This series has also been squeezed into D47633 to provide an overall view.

Commit message:
So that administrators can more easily know what the problem is with the
rules they are trying to set.

The new sysctl 'security.mac.do.print_parse_error' controls whether
trying to set sysctl 'security.mac.do.rules' with invalid rules triggers
printing of the error on the system console.

Setting jail parameters directlty reports an error to the calling
process thanks to the VFS options mechanism used by the jail machinery,
so is not controlled by the new sysctl setting.

[olce]

Rename alloc_parse_error() to make_parse_error() (as it is a constructor really, not just an allocator).

Fix style (v & FLAG => (v & FLAG) != 0 and similar).

In coalesce_id_flags(), add an error message for the new error introduced in D47616 and tweak the existing one.