Added warning when there is no fsync() system call available.
SDBM: added strategic fdatasync() calls to better recover from system crashes.
Increased maximum download buffer size to 1 MiB and default to 128 KiB.

The aim is to reduce fragmentation on the filesystem as much as possible.
Since modern GTKG uses a default request size of 1 MiB, it makes sense to
have it all buffered in RAM before being flushed to disk.
Improved sort routine benchmarking.

Spend more time to do the benchmark, especially on faster machines, to make
sure we measure something accurate with enough sampling from the kernel.

Also introduced prioritization among the various sort routines, to prefer
the ones which we supply over libc's in case they perform equally well.
Fixed typo in comment.
PARQ: accept to move an upload to the "dead" queue even if we wait for QUEUE.

An upload could be flagged as waiting for a QUEUE message, yet there are so
many waiting uploads from that host that we're not contacting it back quickly
enough and all these slots remain "alive" in the queue, preventing real uploads
from starting.

To keep some fairness in the system, we need to expire these uplaods, keeping
them in the "dead" list for a while.   They won't be totally lost until we
run out of PARQ slots, but they will allow newer uploads to start.
Undefine stat and fstat when compiling with gcc-4.7 on Windows.
Windows: moved definition of WINVER from lib/mingw32.h to common.h.
Regenerated Configure for proper Windows compilation with newest MinGW.