SERVER-13712 smoke.py --clean-every=N option

(cherry picked from commit 8cd645a8fa0cce6313c4a15e4f3ec4fd8ac2787e)
SERVER-13712 cleanbb should clean entire /data/db dir

Invoke cleanbb via function call, not subprocess.

clean_dbroot() now cleans entire /data/db if --with-cleanbb specified,
including periodic cleanups.

(cherry picked from commit ab47b0b217ab40971a928bbe3d98bd315bbba716)
SERVER-13712 fix fedora8 builders (python 2.5.1)

(cherry picked from commit 630944421eebbbdbde600b6294e28ec992fc3fba)
SERVER-13724 Add --disable-warnings-as-errors scons option

This change was prompted by attempts to build 2.6.0 in SLES 10 with
its system gcc version 4.1.2. It builds fine other than a sole
warning - src/third_party/boost/boost/thread/detail/thread.hpp:316:
warning: type attributes are honored only at type definition. It
should also help in cases where compilers are too new.
(cherry picked from commit 27c128d303d9d3bcdf471d4da69bd1b8940fd038)
Merge remote-tracking branch 'upstream/v2.6' into v2.6