Add module-specific data support. This adds support for a safe form of `pam_get_data` and `pam_set_data`, where data is (as best as humanly possible) type-safe and restricted to only the module where it was created.

# `libpam-sys-helpers`: Cross-platform helpers for `libpam-sys`

This provides memory-management helpers for `libpam-sys`.

This does not actually depend upon `libpam-sys`; it only uses `libpam-sys-consts`.