Add optional LD_PRELOAD library

Defines a new libunifyfs_preload_gotcha.so library to intercept file system calls from shell commands.

To build this library, add the --enable-preload configure option.

To use this library at runtime, the UnifyFS server should be started as normal. Then in the client, set LD_PRELOAD to point to the library install location. For example, a bash user can set:

export LD_PRELOAD=<unifyfs_install>/lib/libunifyfs_preload_gotcha.so

One may then run shell commands to interact with UnifyFS files, e.g.,:

touch /unifyfs/file1
cp -pr /unifyfs/file1 /unifyfs/file2
ls -l /unifyfs/file1
stat /unifyfs/file1
rm /unifyfs/file1

Due to the variety and variation of I/O functions that may be called by different commands, there is no guarantee that a given invocation is supported under UnifyFS semantics. This feature is experimental, and it should be used at your own risk.

Description

Motivation and Context

How Has This Been Tested?

Types of changes

[ ] Bug fix (non-breaking change which fixes an issue)
[x] New feature (non-breaking change which adds functionality)
[ ] Performance enhancement (non-breaking change which improves efficiency)
[ ] Code cleanup (non-breaking change which makes code smaller or more readable)
[ ] Breaking change (fix or feature that would cause existing functionality to change)
[ ] Testing (addition of new tests or update to current tests)
[ ] Documentation (a change to man pages or other documentation)

Checklist:

[x] My code follows the UnifyFS code style requirements.
[ ] I have updated the documentation accordingly.
[x] I have read the CONTRIBUTING document.
[ ] I have added tests to cover my changes.
[ ] All new and existing tests passed.
[ ] All commit messages are properly formatted.

LLNL / UnifyFS