This changes a boolean param into a proper bitflag field.
Given this only defines a single flag for now this doesn't look like
much of an improvement. But we'll add another flag shortly, where it
starts to make more sense.
Instead of having fopen_temporary() create the file either next
to an existing file or in tmp/, let's split this up clearly into
two different functions, one for creating temporary files next to
existing files, and one for creating a temporary file in a directory.
This splits out a bunch of functions from fileio.c that have to do with
temporary files. Simply to make the header files a bit shorter, and to
group things more nicely.
No code changes, just some rearranging of source files.