dgram: introduce `reuseAddr` option

Introduce new signature for both `dgram.createSocket` method and
`dgram.Socket` constructor:

    dgram.createSocket(options, [listener])

Options should contain `type` property and may contain `reuseAddr`
property. When `reuseAddr` is `true` - SO_REUSEADDR will be issued on
socket on bind.

fix #7415

Signed-off-by: Fedor Indutny <fedor@indutny.com>
test: add `reuseAddr` in dgram-multicast...

Add `reuseAddr` option in `test-dgram-multicast-multi-process.js`
test: remove vm-infinite-recursion

It doesn't work reliably on all platforms.

see #7432

Signed-off-by: Fedor Indutny <fedor@indutny.com>
buffer: truncate buffer after string decode

When our estimates for a storage size are higher than the actual length
of decoded data, the destination buffer should be truncated. Otherwise
`Buffer::Length` will give misleading information to C++ layer.

fix #7365

Signed-off-by: Fedor Indutny <fedor@indutny.com>
buffer: return uint if MSB is 1 in readUInt32

Fix issue where a signed integer is returned.

Example:

var b = new Buffer(4);
b.writeUInt32BE(0xffffffff);
b.readUInt32BE(0) == -1

Signed-off-by: Trevor Norris <trev.norris@gmail.com>