os: implement File.seek/2 and File.tell/0

vlib/os/file.c.v

@@ -16,6 +16,7 @@ struct FileInfo {
 fn C.fseeko(&C.FILE, u64, int) int
 
 fn C._fseeki64(&C.FILE, u64, int) int
+fn C._ftelli64(&C.FILE) i64
 
 fn C.getc(&C.FILE) int
 
@@ -727,3 +728,60 @@ pub fn (mut f File) write_raw_at<T>(t &T, pos u64) ? {
 		return error_with_code('incomplete struct write', nbytes)
 	}
 }
+
+pub enum SeekMode {
+	start
+	current
+	end
+}
+
+// seek moves the file cursor (if any) associated with a file
+// to a new location, offset `pos` bytes from the origin. The origin
+// is dependent on the `mode` and can be:
+//   .start   -> the origin is the start of the file
+//   .current -> the current position/cursor in the file
+//   .end     -> the end of the file
+// If the file is not seek-able, or an error occures, the error will
+// be returned to the caller.
+// A successful call to the fseek() function clears the end-of-file
+// indicator for the file.
+pub fn (mut f File) seek(pos i64, mode SeekMode) ? {
+	if !f.is_opened {
+		return error_file_not_opened()
+	}
+	whence := int(mode)
+	mut res := 0
+	$if x64 {
+		$if windows {
+			res = C._fseeki64(f.cfile, pos, whence)
+		} $else {
+			res = C.fseeko(f.cfile, pos, whence)
+		}
+	}
+	$if x32 {
+		res = C.fseek(f.cfile, pos, whence)
+	}
+	if res == -1 {
+		return error(posix_get_error_msg(C.errno))
+	}
+}
+
+// tell will return the current offset of the file cursor measured from
+// the start of the file, in bytes. It is complementary to seek, i.e.
+// you can use the return value as the `pos` parameter to .seek( pos, .start ),
+// so that your next read will happen from the same place.
+pub fn (f &File) tell() ?i64 {
+	if !f.is_opened {
+		return error_file_not_opened()
+	}
+	mut pos := i64(0)
+	$if windows && x64 {
+		pos = C._ftelli64(f.cfile)
+	} $else {
+		pos = C.ftell(f.cfile)
+	}
+	if pos == -1 {
+		return error(posix_get_error_msg(C.errno))
+	}
+	return pos
+}