cleaner separation of posix client code and library api

[MichaelBrim]

Description

This PR includes code cleanup and restructuring of the client libraries to make sure the library API (i.e., libunifyfs_api) does not rely on any of the POSIX I/O wrapping code. The support for file transfers in the library API has also been updated to use a new server-based transfer scheme where servers are responsible for writing their local data to the destination file. A summary of the various cleanup/changes follows:

client changes:

• remove unused app_id parameter to unifyfs_mount()
• Makefile updates to clearly indicate source files corresponding to
  core functionality, library API, and POSIX I/O support
• adds unifyfs_client state structure to avoid use of global variables
• move implementation of client API functions to client_api.c
• move wrapper macros to unifyfs_wrap.h
• introduces posix_client.[ch] to manage global state for POSIX I/O
• add unifyfs_fid.[ch] to contain all unifyfs_fid_xxx() functions
• PMPI wrapper for unifyfs_mount() now checks for and uses environment
  variable setting for UNIFYFS_MOUNTPOINT
• library API transfer support now uses server-based transfers
• adds checks in library API to ensure created/accessed files are within
  client namespace

common changes:

• arraylist_add() now returns position of appended item
• remove MDHIM-specific configuration settings
• remove unused constants from unifyfs_const.h

server changes:

• implements support for parallel transfers where each server with
  local file data copies it to the destination file

tests/examples changes:

• use updated client and library APIs
• new library API transfer test
• add library API support to examples testutil.h
• new write-transfer example
• fix sharness.sh test_dir_is_empty() to ignore trailing slashes

How Has This Been Tested?

Tested in Ubuntu Docker environment and on OLCF Summit

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• 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:

• My code follows the UnifyFS code style requirements.
• I have updated the documentation accordingly.
• 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.