add getentropy function

based loosely on patch by Hauke Mehrtens; converted to wrap the public
API of the underlying getrandom function rather than direct syscalls,
so that if/when a fallback implementation of getrandom is added it
will automatically get picked up by getentropy too.

include/unistd.h

@@ -176,6 +176,7 @@ int acct(const char *);
 long syscall(long, ...);
 int execvpe(const char *, char *const [], char *const []);
 int issetugid(void);
+int getentropy(void *, size_t);
 #endif
 
 #ifdef _GNU_SOURCE

src/misc/getentropy.c

+#include <sys/random.h>
+#include <pthread.h>
+#include <errno.h>
+
+int getentropy(void *buffer, size_t len)
+{
+	int cs, ret;
+	char *pos = buffer;
+
+	if (len > 256) {
+		errno = EIO;
+		return -1;
+	}
+
+	pthread_setcancelstate(PTHREAD_CANCEL_DISABLE, &cs);
+
+	while (len) {
+		ret = getrandom(pos, len, 0);
+		if (ret < 0) {
+			if (errno == EINTR) continue;
+			else break;
+		}
+		pos += ret;
+		len -= ret;
+		ret = 0;
+	}
+
+	pthread_setcancelstate(cs, 0);
+
+	return ret;
+}