parallelized regen

Script enabling regen in parallel.

Stats are gathered from-

Hardware: Compaq Proliant 6400R 4x500Mhz Pentium3 / Xeon, 2MB Cache 1GB RAM

Finally, PORTDIR was tmpfs mounted to try and minimize any IO influence beyond expected cache updates.

Comparison is against portage-2.1_pre7, and rev 864 of pkgcore.

commandrealusersys
emerge --regen276m28.993s151m59.860s84m52.690s
python smp-regen.py 1166m48.433s118m18.340s46m6.630s
python smp-regen.py 285m40.726s4m29.270s0m52.450s
python smp-regen.py 359m0.185s4m34.640s0m56.790s
python smp-regen.py 446m9.578s4m45.900s1m2.220s