clock_gettime: Add Linux aliases for CLOCK_*

Linux standardized what we call CLOCK_{REALTIME,MONOTONIC}_FAST as
CLOCK_{REALTIME,MONOTONIC}_COARSE. In addition, Linux spells
CLOCK_UPTIME as CLOCK_BOOTTIME.

Add aliases to time.h and document these new aliases in
clock_gettime(2).

Reviewed by:		vangyzen, kib (prior), dchagin (prior)
Sponsored by:		Netflix
Differential Revision:	https://reviews.freebsd.org/D30988

lib/libc/sys/clock_gettime.2

@@ -29,7 +29,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd May 13, 2021
+.Dd July 1, 2021
 .Dt CLOCK_GETTIME 2
 .Os
 .Sh NAME
@@ -68,14 +68,17 @@ as well as the following values:
 .It Dv CLOCK_REALTIME
 .It Dv CLOCK_REALTIME_PRECISE
 .It Dv CLOCK_REALTIME_FAST
+.It Dv CLOCK_REALTIME_COARSE
 Increments as a wall clock should.
 .It Dv CLOCK_MONOTONIC
 .It Dv CLOCK_MONOTONIC_PRECISE
 .It Dv CLOCK_MONOTONIC_FAST
+.It Dv CLOCK_MONOTONIC_COARSE
 Increments in SI seconds.
 .It Dv CLOCK_UPTIME
 .It Dv CLOCK_UPTIME_PRECISE
 .It Dv CLOCK_UPTIME_FAST
+.It Dv CLOCK_BOOTTIME
 Starts at zero when the kernel boots and increments
 monotonically in SI seconds while the machine is running.
 .It Dv CLOCK_VIRTUAL
@@ -104,6 +107,16 @@ Similarly,
 .Fa CLOCK_UPTIME_PRECISE
 are used to get the most exact value as possible, at the expense of
 execution time.
+The clock IDs
+.Fa CLOCK_REALTIME_COARSE ,
+.Fa CLOCK_MONOTONIC_COARSE
+are aliases of corresponding IDs with _FAST suffix for compatibility with other
+systems.
+Finally,
+.Dv CLOCK_BOOTTIME
+is an alias for
+.Dv CLOCK_UPTIME
+for compatibility with other systems.
 .Pp
 The structure pointed to by
 .Fa tp

sys/sys/_clock_id.h

@@ -85,6 +85,15 @@
 #define	CLOCK_PROCESS_CPUTIME_ID 15
 #endif /* __POSIX_VISIBLE >= 199309 */
 
+/*
+ * Linux compatible names.
+ */
+#if __BSD_VISIBLE
+#define	CLOCK_BOOTTIME		CLOCK_UPTIME
+#define	CLOCK_REALTIME_COARSE	CLOCK_REALTIME_FAST
+#define	CLOCK_MONOTONIC_COARSE	CLOCK_MONOTONIC_FAST
+#endif
+
 #if __BSD_VISIBLE
 #define TIMER_RELTIME	0x0	/* relative timer */
 #endif