Fix macOS build and getopt crash

[jimscard]

Summary:

• fix the macFUSE 3 Darwin callback signatures used by dislocker-fuse
• terminate the getopt_long option table correctly to avoid a help-path crash

Details:
On macOS, recent macFUSE headers use Darwin-specific callback signatures for getattr and readdir, so the existing Linux-style function pointers fail to compile. This change adds Apple-specific signatures and field access while preserving existing behavior on other platforms.

The option parsing code also built the long option table without the required trailing zero entry. getopt_long could then read past the end of the array and crash even on simple invocations like dislocker-fuse -h. Using calloc for one extra zeroed entry preserves the existing options and provides the required terminator.

Validation:

• make
• make travis-test