Fix multi-block reorg transaction resurrection.

Fix non-standard disconnected transactions causing mempool orphans.
This bug could cause nodes running with the -debug flag to crash.

OSX: use 'FD_FULLSYNC' with LevelDB, which will (hopefully!)
prevent the database corruption issues many people have
experienced on OSX.
Linux: clicking on bitcoin: links was broken if you were using
a Gnome-based desktop.
Fix a hang-at-shutdown bug that only affects users that compile
their own version of Bitcoin against Boost versions 1.50-1.52.

Checkpoint at block 250,000 to speed up initial block downloads
and make the progress indicator when downloading more accurate.
Thanks to everybody who contributed to the 0.8.4 releases!