Pass an extra argument to the function returning the particle's softening. Removed the field from the gpart themselves.

Note also needs --enable-compiler-warnings=yes and --disable-vec and isn't known to run!

Also detect whether the current POSIX implementation offers the optional function posix_fallocate(). If not, switch off the particle streamed logging.

added logger_log_part, still not sure how to handle/store old offsets, so leaving that to the caller.