fileio: Refactor errmsg handling

Adds os_strerror() result to a number of places. Also since I could not track 
where err\* variables are NULL and where they are not, using macros to make sure 
that all three variables are set at once.

Removes #ifdef UNIX around the use of os_fsync, makes it use os_close in place 
of close in some places.

runtime/doc/options.txt

@@ -2740,6 +2740,9 @@ A jump table for the options with a short description can be found at |Q_op|.
 	mode, so it may be undesirable in some situations.  Be warned that
 	turning this off increases the chances of data loss after a crash.
 
+	Currently applies only to writing the buffer with e.g. |:w| and 
+	|writefile()|.
+
 				   *'gdefault'* *'gd'* *'nogdefault'* *'nogd'*
 'gdefault' 'gd'		boolean	(default off)
 			global