Use absolute paths

[Stebalien]

When looking into #40, I realized we weren't actually doing what I claimed. Except for never-named tempfiles on Linux and auto-removed temporary files on Windows, we need to use absolute path names to guard against changes in the current working directory.

Unfortunately, this is a [breaking change] on some unix-like OSs. It's possible to:

1. Chdir to a directory.
2. Chmod the parent directory to remove the search (x) permission.

This will break any uses of absolute path names. The solution is the one described in #40: open the current directory, use openat on the relative path, and then close the current directory. However:

1. This will only help with unnamed temporary files unless we're willing to hold the file directory open (probably not the best idea).
2. This will require two filesystem extra syscalls (instead of one as with getcwd).

[Stebalien]

I snuck in two other small fixes while I was at it (second two commits).

[Stebalien]

I snuck in two other small fixes while I was at it (second two commits).

[KodrAus]

This looks good to me!