uarea swap-out

[Mindaugas Rasiukevicius <rmind%netbsd.org@localhost>]

Hello,

The following patch removes the uarea swap-out functionality:

http://www.netbsd.org/~rmind/uarea_swapout_rem.diff

After some thought and discussion with few developers, I believe it is
worth, because it:

- Addresses the issue described in PR/38828.
- Gets small, but nice simplification in threading code.
- Avoids few scans on LWP list and thus potentially long holds of proc_lock.
- Removes __SWAP_BROKEN cases.
- Cuts around 1000 lines of code.

Concerns?

-- 
Mindaugas