swap reservation accounting

[Kostik Belousov]

Hello everyone,

I developed the patch for 7-CURRENT (also should apply cleanly to 6)
that implements swap reservation accounting and allows to cap the
allocation of anonymous memory to the available swap. Effectively,
this allows to optionally turn off overcommit feature of the VM.

Besides this, per-user swap reservation limits could be
enforced. Corresponding capability is added to the login.conf database.

For some description of the implementation, TODO list and patch
itself, please see http://kostikbel.narod.ru

This is my first touch of the FreeBSD code, you valuable comments and
reviews would be greatly appreciated.

Best regards,
Kostik Belousov