privops: add method PRV_AdjustFreq()

In preparation of adding OpenBSD support:

OpenBSD doesn't support timex but adjusting frequency using the
adjfreq(2) system call. Add a privops method PRV_AdjustFreq() to allow
unprivileged processes to set the time.

privops.h

@@ -64,6 +64,12 @@ void PRV_ReloadDNS(void);
 #define PRV_ReloadDNS DNS_Reload
 #endif
 
+#ifdef PRIVOPS_ADJUSTFREQ
+int PRV_AdjustFreq(const int64_t *freq, int64_t *oldfreq);
+#else
+#define PRV_AdjustFreq adjfreq
+#endif
+
 #ifdef PRIVOPS_HELPER
 void PRV_Initialise(int scfilter_level);
 void PRV_StartHelper(void);