Use mkdir() and rmdir() rather than open() and unlink() to create the lock

files with the unix-dotlock VFS.  The change is backwards compatible and,
we are told, works better on some network filesystems.

FossilOrigin-Name: e2f08426d7a84a2ac6148f485b7af377201a175b

test/syscall.test

@@ -59,7 +59,7 @@ do_test 2.1.2 { test_syscall exists nosuchcall } 0
 foreach s {
     open close access getcwd stat fstat ftruncate
     fcntl read pread write pwrite fchmod fallocate
-    pread64 pwrite64 unlink openDirectory
+    pread64 pwrite64 unlink openDirectory mkdir rmdir
 } {
   if {[test_syscall exists $s]} {lappend syscall_list $s}
 }