Fix a whole bunch of #includes that were either wrong or redundant.

The first rule of portability for us is 'thou shalt have no other gods
before c.h', and a whole lot of these files were either not including
c.h at all, or including random system headers beforehand, either of
which sins can mess up largefile support nicely.  Once you have
included c.h, there is no need to re-include what it includes, either.