Use fixed-width reads for Btrieve file fields

[GregoryMorse]

This fixes LP64 portability issues when reading Btrieve on-disk fields.

The current code uses long / unsigned long and pointer casts for several serialized 32-bit fields. That works on Win32/Win64 because long is 32-bit there, but on Linux/macOS long is 64-bit, so packed structures and direct casts can read the wrong bytes.

Changes:

• Use <stdint.h> fixed-width types for serialized page IDs and constants.
• Add little-endian 16/32-bit read helpers for disk fields.
• Replace host-sized pointer casts for FCR record counts, page IDs, page headers, PAT page pointers, and record headers.
• Preserve the existing public API and behavior.

Validation:

• Built BtrieveFileSaver.vcxproj with VS2022, Release|Win32.

[GregoryMorse]

Just to note - this has been tested independently now on Windows, Linux, MacOS by just statically compiling against the BtrieveFileSaverLib.c and its header files.