osdep/io: move I/O utility functions to misc/io_utils

The purpose of osdep/io is to provide a wrapper for POSIX I/O functions
on Windows. The last 2 functions are utility functions which don't serve
this purpose.
Move them to a separate file.

meson.build

@@ -134,6 +134,7 @@ sources = files(
     'misc/bstr.c',
     'misc/charset_conv.c',
     'misc/dispatch.c',
+    'misc/io_utils.c',
     'misc/json.c',
     'misc/language.c',
     'misc/natural_sort.c',