Increase supported VDSO maximum size to 64K.

On 3.16.3-200.fc20.x86_64 the VSDO size for 64-bit processes is 8K.
Rename monkeypatch_vdso to monkey_patch_vdso_after_preload_init.
Refactor monkeypatching code to have read_vdso_symbols return two std::vectors.

This avoids passing four variables around and lets us read the VDSO symbols
near where we use them.
Monkeypatch x86-64 VDSO syscalls immediately after exec.

The shared-lib constructor in the 'constructor' test runs before librrpreload
has initialized, and calls the VDSO syscall 'gettimeofday'. If that happens
before the VDSO code is patched, we'll diverge during replay. By patching
the VDSO syscalls immediately after exec there's no way they can be called
before being patched.