Fix the longstanding limit on 32 bits that allowed us to allocate only 2GB of ram by moving the big malloc calls to mmap equivalents which allow us to mmap up to 2^44 bytes of anonymous space.
Use progressively smaller preallocation to try and defragment ram prior to real mmap call to increase success rate of allocating ram when it's a significant proportion of total ram.
Don't fail if preallocation is unsuccessful.
Add more detailed error reporting.
Minor cleanups.
This will increase speed of compression and generate a smaller file, but not be backward compatible.
Tweak the way memory is allocated to optimise chances of success and minimise slowdown for the machine.
fsync to empty dirty data before allocating large ram to increase chance of mem allocation and decrease disk thrash of write vs read.
Add lots more information to verbose mode.
Lots of code tidying and minor tweaks.
This reverts commit 603785f783.
Not everyone has a large /tmp directory and may not be able to fit the temporary files in /tmp so it's not a great idea to do this by default.
