This is a dummy implementation for performance comparison with the FUSE
filesystem ciopfs.  It converts names to lowercase on-disk, so it is not
stable, but it is what gives the best performance.  In comparison to
ciopfs, it performs 10x faster when doing inexact-case searches than
CIOPFS.  For case-exact matches, they perform pretty much the same,
which is pretty much the same as a flat access to the filesystem,
i.e. we won't  get any better on that case.

But, it has the same issues as CIOPFS:

  - Not stable names
  - Uppercase Files created outside the CI mountpoint are hidden.
  - Doesn't handle utf8.

This is a very trivial implementation, but one which demonstrates the
advantages of syscall_intercept over FUSE approaches for this basic
scenario.

The benchmark I used for the above was pathwalk, which does a simple,
create, randomize pathname case and search. For 10k files, The following
times were collected.

=== CI lookups ===

* libcasefold test:  inexact match of 10k files
LD_PRELOAD=libcasefold.so ./pathwalkci /tmp/ci/ 10000 ci
created 10000 files
Performing case inexact lookup
found 10000 files. TIME: 0.011

* CIOPS test:  inexact match of 10k files
./pathwalkci /tmp/ciopfs-ci/ 10000 ci
[git:master@pathwalkci]
created 10000 files
Performing case inexact lookup
found 10000 files. TIME: 0.118

=== CS lookups ===

* libcasefold test:  Exact matches of 10k files
LD_PRELOAD=libcasefold.so   ./pathwalkci /tmp/ci/ 10000
created 10000 files
Performing case exact lookup
found 10000 files. TIME: 0.011

* CIOPS test:  exact match of 10k files
./pathwalkci /tmp/ciopfs-ci/ 10000
created 10000 files
Performing case exact lookup
found 10000 files. TIME: 0.014

* Flat tests: Exact match of 10k files
./pathwalkci /tmp/ci/ 10000
created 10000 files
Performing case exact lookup
found 10000 files. TIME: 0.009

Signed-off-by: Gabriel Krisman Bertazi <krisman@collabora.co.uk>