patch 8.1.0915: fsync() may not work properly on Mac

Problem:    fsync() may not work properly on Mac.
Solution:   Use fcntl() with F_FULLFSYNC. (suggested by Justin M. Keyes)
diff --git a/src/memfile.c b/src/memfile.c
index f2e774a..1a52734 100644
--- a/src/memfile.c
+++ b/src/memfile.c
@@ -600,7 +600,7 @@
 	 */
 	if (STRCMP(p_sws, "fsync") == 0)
 	{
-	    if (fsync(mfp->mf_fd))
+	    if (vim_fsync(mfp->mf_fd))
 		status = FAIL;
 	}
 	else
@@ -617,7 +617,7 @@
 #ifdef VMS
 	if (STRCMP(p_sws, "fsync") == 0)
 	{
-	    if (fsync(mfp->mf_fd))
+	    if (vim_fsync(mfp->mf_fd))
 		status = FAIL;
 	}
 #endif
@@ -627,7 +627,7 @@
 #endif
 #ifdef AMIGA
 # if defined(__AROS__) || defined(__amigaos4__)
-	if (fsync(mfp->mf_fd) != 0)
+	if (vim_fsync(mfp->mf_fd) != 0)
 	    status = FAIL;
 # else
 	/*