fix(fuse,scripts): log why a create finds no parent and fit the metadata floor to FUSE

A create that answered ENOENT left no trace in the log. It now warns
when the parent folder is no longer known, and when the parent left the
tree while its children were being listed, naming the parent inode and
the new name.

The acceptance script's metadata throughput floor was 20 operations per
second, which a FUSE mount cannot reach: each create and unlink waits for
Drive before it returns, about 350 ms apiece. On a FUSE mount the floor
is now 3 operations per second; other paths keep 20. The mount type is
read from /proc/self/mountinfo.

docs/TESTING.md

@@ -97,7 +97,8 @@ Besides the syscall contract, the suite covers:
 - **Throughput.** 64 MiB written and read sequentially, 1 MiB in 4 KiB writes,
   and metadata operations on 100 files. The rates are printed. The case fails
   below `PDFS_ACCEPTANCE_MIN_MIBPS` (default 10) or `PDFS_ACCEPTANCE_MIN_OPS`
-  (default 20 operations per second).
+  (default 20 operations per second, or 3 on a FUSE mount, where each create
+  and unlink waits for Drive before it returns).
 
 ### The mount is diffed against an ordinary filesystem