sys: introduce generic driver

This driver is intended to complete system-specific drivers that don't
have implemented all required driver functionality. Currently, it
implements offset functions working on top of system-specific frequency
functions. Offsets are corrected by changing frequency, similarly to
fast slewing implemented in the Linux driver.

sys_generic.h

@@ -0,0 +1,42 @@
+/*
+  chronyd/chronyc - Programs for keeping computer clocks accurate.
+
+ **********************************************************************
+ * Copyright (C) Miroslav Lichvar  2014
+ * 
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of version 2 of the GNU General Public License as
+ * published by the Free Software Foundation.
+ * 
+ * This program is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * General Public License for more details.
+ * 
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
+ * 
+ **********************************************************************
+
+  =======================================================================
+
+  Header file for generic driver
+  */
+
+#ifndef GOT_SYS_GENERIC_H
+#define GOT_SYS_GENERIC_H
+
+#include "localp.h"
+
+/* Register a completed driver that implements offset functions on top of
+   provided frequency functions */
+extern void SYS_Generic_CompleteFreqDriver(double max_set_freq_ppm, double max_set_freq_delay,
+                                           lcl_ReadFrequencyDriver sys_read_freq,
+                                           lcl_SetFrequencyDriver sys_set_freq,
+                                           lcl_ApplyStepOffsetDriver sys_apply_step_offset,
+                                           lcl_SetLeapDriver sys_set_leap);
+
+extern void SYS_Generic_Finalise(void);
+
+#endif  /* GOT_SYS_GENERIC_H */