Cleaned up the structure of the whole OS-abstraction layer.

fileops.c now contains a set of utility methods for file management used
by the library. These are abstractions on top of the original POSIX
calls.

There's a new file called `posix.c` that contains
emulations/reimplementations of all the POSIX calls the library uses.
These are prefixed with `p_`. There's a specific posix file for each
platform (win32 and unix).

All the path-related methods have been moved from `utils.c` to `path.c`
and have their own prefix.

For information on FlushFileBuffers(), see the msdn document
at msdn.microsoft.com/en-us/library/aa364439(VS.85).aspx

Note that Windows 2000 is shown as the minimum windows version
to support FlushFileBuffers(), so if we wish to support Win9X
and NT4, we will need to add code to dynamically check if
kernel32.dll contains the function.

The only error return mentioned in the msdn document is
ERROR_INVALID_HANDLE, which is returned if the file/device
(eg console) is not buffered. The fsync(2) manpage says that
EINVAL is returned in errno, if "fd is bound to a special
file which does not support synchronization".

Signed-off-by: Ramsay Jones <ramsay@ramsay1.demon.co.uk>

Signed-off-by: Ramsay Jones <ramsay@ramsay1.demon.co.uk>
Signed-off-by: Andreas Ericsson <ae@op5.se>