buffer: support aligned allocations

[ronag]

Adds an optional alignment argument to Buffer.allocUnsafe() and
Buffer.allocUnsafeSlow(), guaranteeing that the memory backing the returned
buffer starts at an address that is a multiple of alignment.

Why

Some OS interfaces refuse to work with unaligned memory. The motivating case is
unbuffered ("direct") file I/O: a read or write on a descriptor opened with
O_DIRECT fails with EINVAL unless the buffer address is a multiple of the
device's logical block size. There was previously no way to get such a buffer
from JS — the address of a backing store can neither be observed nor chosen.
Alignment is also useful purely for performance, e.g. keeping a hot buffer from
straddling one more cache line than its size requires.

How

V8 does not allow picking the address of a backing store, so alignment is
achieved by over-allocating alignment - 1 bytes and positioning the buffer at
the first suitably aligned byte within them. A new arrayBufferAlignedOffset()
binding computes that offset.

It returns an offset rather than a finished Buffer for two reasons: external
backing stores are rejected outright when the V8 sandbox is enabled, and
Buffer::New() is not usable during buffer.js evaluation — the buffer
prototype is only wired up after require('buffer') returns, which the pool
creation runs before.

The Buffer.allocUnsafe() pool is now cache-line aligned itself, so pooled
allocations can satisfy any alignment up to 64 bytes by padding their offset
into the pool instead of allocating separately. poolOffset is therefore now
tracked relative to a new poolBase.

Status

• Not built or tested. eslint (lib + doc samples) and cpplint pass;
  lint-md reports only the expected pr-url: REPLACEME warnings.
• Doc changes: entries still need the real PR number.