Fix reference count region initialization

[rbx]

Fix a bug where the externally created region's rc segment settings would be ignored by the region user. The changes make sure that the registered size value is honored in case:

• region user creates it itself.
• region config is created externally (region user accepts external config value).
• region undergoes soft reset and is recreated by the region user (in this case the configured value still must be honored).

Other additions:

• Monitor output for the reference count segment to include the amount of free space in the segment.
• example script for usage with externally created regions.

[coderabbitai]

Walkthrough

Walkthrough

The changes across various scripts and source files are primarily focused on configuration and initialization improvements in a messaging system setup. Bash scripts have been updated to handle environment variables, command-line arguments, and default values more efficiently. CMake configuration now uses the @ONLY option to prevent variable expansion. In the C++ source files, there have been adjustments to the shared memory management, including the removal of redundant variables and the addition of initialization steps for reference counting segments.

Changes

File	Summary
examples/.../fairmq-start-ex-region-advanced-external.sh.in	Updated to set FAIRMQ_PATH, adjust transport type and message size based on user input, and configure processes with specific shared memory and channel parameters.
examples/.../fairmq-start-ex-region-advanced.sh.in	Changed to assign msgSize using the second input parameter and simplified by using default value assignments for transport and msgSize.
examples/.../fairmq-start-ex-region.sh.in	Modified to assign msgSize using the second input parameter and introduced parameter expansion for default values.
examples/CMakeLists.txt	Added @ONLY option to configure_file function calls to prevent CMake variable processing.
fairmq/shmem/Manager.h	Removed rcSegmentSize variable, added checks and settings for rcSegmentSize in cfg based on shared memory regions.
fairmq/shmem/Monitor.cxx	Enhanced Monitor::PrintShm function to include free memory output of rcCountSegment.
fairmq/shmem/UnmanagedRegion.h	Added InitializeRefCountSegment call after zeroing free memory in the initialization process.

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