AthAllocators, etc: Reduce memory requirements.

Fix error reporting when mprotect() fails --- should use errno, not the return value.

Allow ArenaBlockAllocatorBase::reserve() to allocate blocks smaller than the originally requested block size.

When using DataVectorWithAlloc, call reserve on it before filling in order to minimize wasted memory.

May help with ATR-24374 / ATLASRECTS-6630 / ATLASRECTS-6629.