sys_posix: support SCHED_FIFO and mlockall on more OSs

Real-time scheduling and memory locking is available on posix compliant
OSs. This patch centralizes this functionality and brings support to
FreeBSD, NetBSD, and Solaris.

[ML: updated coding style]

sys_posix.h

@@ -0,0 +1,36 @@
+/*
+  chronyd/chronyc - Programs for keeping computer clocks accurate.
+
+ **********************************************************************
+ * Copyright (C) Richard P. Curnow  1997-2003
+ * Copyright (C) John G. Hasler  2009
+ * Copyright (C) Miroslav Lichvar  2009-2012, 2014-2018
+ *
+ * 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.
+ *
+ **********************************************************************
+
+  =======================================================================
+
+  The header file for shared Posix functionality
+  */
+
+#ifndef GOT_SYS_POSIX_H
+#define GOT_SYS_POSIX_H
+
+extern void SYS_Posix_MemLockAll(void);
+
+extern void SYS_Posix_SetScheduler(int priority);
+
+#endif  /* GOT_SYS_POSIX_H */