This fixes a load-time failure on versions of Windows too old to have
that function in kernel32.dll.
We use it to determine whether a file was safe to overwrite in the
context of PuTTY session logging: if it's safe, we skip the 'do you
want to overwrite or append?' dialog box.
On earlier Windows you can use FindFirstFile to get a similar effect,
so that's what we fall back to. It's not quite the same, though - if
you pass a wildcard then it will succeed when you'd rather it had
failed. But it's good enough to at least work in normal cases.
Now that the new CMake build system is encouraging us to lay out the
code like a set of libraries, it seems like a good idea to make them
look more _like_ libraries, by putting things into separate modules as
far as possible.
This fixes several previous annoyances in which you had to link
against some object in order to get a function you needed, but that
object also contained other functions you didn't need which included
link-time symbol references you didn't want to have to deal with. The
usual offender was subsidiary supporting programs including misc.c for
some innocuous function and then finding they had to deal with the
requirements of buildinfo().
This big reorganisation introduces three new subdirectories called
'utils', one at the top level and one in each platform subdir. In each
case, the directory contains basically the same files that were
previously placed in the 'utils' build-time library, except that the
ones that were extremely miscellaneous (misc.c, utils.c, uxmisc.c,
winmisc.c, winmiscs.c, winutils.c) have been split up into much
smaller pieces.
