Removed some obsolete macros and added a few more headers and function
checks.

I really, really, wasn't think when writing commit
91b131a0

Bug introduced in ece21a66
It's amazing how many mistakes one person can make in a few hours.

Bug introduced in 796d043c
Apparently I was still sleeping or something...

And it wasn't even used for a path, but for a row on the screen...

So we're actually back to having one header file for everything,
except it's now maintainable.

Make sure not to pass the absolute root directory to lstat() and
chdir(), as these functions can't handle long path names.

That's what you get when you change the way a function works...

ncdu already does seem to handle longer paths now, though there are
still a few places where the code still relies on PATH_MAX. Will fix
those later.

This is the first step into replacing all code that relies on PATH_MAX,
more changes will follow.

suseconds_t isn't always defined on some systems, but all possible
values are guaranteed to fit into a long, so use that.

...that was for debugging, which isn't necessary anymore.

This makes deleting multiple files a -lot- easier...