patch 8.1.2094: the fileio.c file is too big

Problem:    The fileio.c file is too big.
Solution:   Move buf_write() to bufwrite.c. (Yegappan Lakshmanan,
            closes #4990)

src/structs.h

@@ -3882,3 +3882,32 @@ typedef struct spat
     int		    no_scs;	// no smartcase for this pattern
     soffset_T	    off;
 } spat_T;
+
+#define WRITEBUFSIZE	8192	// size of normal write buffer
+
+#define FIO_LATIN1	0x01	// convert Latin1
+#define FIO_UTF8	0x02	// convert UTF-8
+#define FIO_UCS2	0x04	// convert UCS-2
+#define FIO_UCS4	0x08	// convert UCS-4
+#define FIO_UTF16	0x10	// convert UTF-16
+#ifdef MSWIN
+# define FIO_CODEPAGE	0x20	// convert MS-Windows codepage
+# define FIO_PUT_CP(x) (((x) & 0xffff) << 16)	// put codepage in top word
+# define FIO_GET_CP(x)	(((x)>>16) & 0xffff)	// get codepage from top word
+#endif
+#ifdef MACOS_CONVERT
+# define FIO_MACROMAN	0x20	// convert MacRoman
+#endif
+#define FIO_ENDIAN_L	0x80	// little endian
+#define FIO_ENCRYPTED	0x1000	// encrypt written bytes
+#define FIO_NOCONVERT	0x2000	// skip encoding conversion
+#define FIO_UCSBOM	0x4000	// check for BOM at start of file
+#define FIO_ALL	-1	// allow all formats
+
+// When converting, a read() or write() may leave some bytes to be converted
+// for the next call.  The value is guessed...
+#define CONV_RESTLEN 30
+
+// We have to guess how much a sequence of bytes may expand when converting
+// with iconv() to be able to allocate a buffer.
+#define ICONV_MULT 8