lib: posix: update usleep() to follow the POSIX spec

The original implementation of `usleep()` was not compliant
to the POSIX spec in 3 ways.
- calling thread may not be suspended (because `k_busy_wait()`
  was previously used for short durations)
- if `usecs` > 1000000, previously we did not return -1 or set
  `errno` to `EINVAL`
- if interrupted, previously we did not return -1 or set
  `errno` to `EINTR`

This change addresses those issues to make `usleep()` more
POSIX-compliant.

Signed-off-by: Chris Friedt <[email protected]>
(cherry picked from commit 7b95428)

Author: Chris Friedt

lib/posix/sleep.c

@@ -4,6 +4,8 @@
  * SPDX-License-Identifier: Apache-2.0
  */
 
+#include <errno.h>
+
 #include <kernel.h>
 #include <posix/unistd.h>
 
@@ -28,10 +30,19 @@ unsigned sleep(unsigned int seconds)
  */
 int usleep(useconds_t useconds)
 {
-	if (useconds < USEC_PER_MSEC) {
-		k_busy_wait(useconds);
-	} else {
-		k_msleep(useconds / USEC_PER_MSEC);
+	int32_t rem;
+
+	if (useconds >= USEC_PER_SEC) {
+		errno = EINVAL;
+		return -1;
+	}
+
+	rem = k_usleep(useconds);
+	__ASSERT_NO_MSG(rem >= 0);
+	if (rem > 0) {
+		/* sleep was interrupted by a call to k_wakeup() */
+		errno = EINTR;
+		return -1;
 	}
 
 	return 0;