fixes for #1078

[qwcode]

fixes mentioned in comments in #1078

the most controversial bit in here is changing the build dir name
(note that we changed the name in 1.3.1 due to security issues, and had no fallout from that)

[qwcode]

@jezdez, as mentioned over IRC, I did some manual testing on this. here's the sequence I went thru:

• started with pip-1.4rc2 (which didn't have this fix)
• validated that pip could not clean a build dir from /tmp/pip-build-root (which didn't have the delete marker file)
• upgraded to tip of 1.4.x
• confirmed it created a new /tmp/pip_build_root (different name) with the delete marker
• then "sudo pip install peppercorn" and confirmed the build dir was cleaned.
• then "sudo pip install --no-clean peppercorn" (which leaves /tmp/pip_build_root/peppercorn)
• then "sudo pip uninstall peppercorn"
• then "sudo pip install peppercorn" and got the PreviousBuildDirError as expected (and /tmp/pip_build_root/peppercorn was present)
• then deleted /tmp/pip_build_root/peppercorn
• then "sudo pip install peppercorn" which worked and /tmp/pip_build_root was deleted after