[#2706] Allow pid up to 4194304

Motivation:

The PID_MAX_LIMIT on 64bit linux systems is 4194304 and on osx it is 99998. At the moment we use 65535 as an upper-limit which is too small.

Modifications:

Use 4194304 as max possible value

Result:

No more false-positives when try to detect current pid.
Fix broken test after change the maximal value of the pid. Part of [#2706]
Use correct exception message when throw exception from native code

Motivation:

We sometimes not use the correct exception message when throw it from the native code.

Modifications:

Fixed the message.

Result:

Correct message in exception
Fix a bug where AbstractConstant.compareTo() returns 0 for different constants

Related issue: #2354

Motivation:

AbstractConstant.compareTo() can return 0 even if the specified constant
object is not the same instance with 'this'.

Modifications:

- Compare the identityHashCode of constant first. If that fails,
  allocate a small direct buffer and use its memory address as a unique
  value.  If the platform does not provide a way to get the memory
  address of a direct buffer, use a thread-local random value.
- Signal cannot extend AbstractConstant. Use delegation.

Result:

It is practically impossible for AbstractConstant.compareTo() to return
0 for different constant objects.
Fix a ConstantPoolTest failure
Remove duplicate range check in AbstractByteBuf.skipBytes()
Use our own URL shortener wherever possible
[#2720] Check if /proc/sys/net/core/somaxconn exists before try to parse it

Motivation:

As /proc/sys/net/core/somaxconn does not exists on non-linux platforms you see a noisy stacktrace when debug level is enabled while the static method of NetUtil is executed.

Modifications:

Check if the file exists before try to parse it.

Result:

Less noisy logging on non-linux platforms.
Remove duplicated code
Improve Bzip2BitReader/Writer

Motivation:

Before this changes Bzip2BitReader and Bzip2BitWriter accessed to ByteBuf byte by byte. So tests for Bzip2 compression codec takes a lot of time if we ran them with paranoid level of resource leak detection. For more information see comments to #2681 and #2689.

Modifications:

- Increased size of bit buffers from 8 to 64 bits.
- Improved reading and writing operations.
- Save link to incoming ByteBuf inside Bzip2BitReader.
- Added methods to check possible readable bits and bytes in Bzip2BitReader.
- Updated Bzip2 classes to use new API of Bzip2BitReader.
- Added new constants to Bzip2Constants.

Result:

Increased size of bit buffers and improved performance of Bzip2 compression codec (for general work by 13% and for tests with paranoid level of resource leak detection by 55%).
Fix buffer leaks in DnsResponseDecoder and DnsResponseDecoderTest

Motivation:

There were two buffer leaks in the codec-dns.

Modifications:

- Fix buffer leak in DnsResponseTest.readResponseTest()
- Correctly release DnsResources on Exception

Result:

No more buffer leaks in the codec-dns module.
[#2718] Added private key decryption to JDK SSL server context.

Motivation:

Currently it is not possible to load an encrypted private key when
creating a JDK based SSL server context.

Modifications:

- Added static method to JdkSslServerContext which handles key spec generation for (encrypted) private keys and make use of it.
-Added tests for creating a SSL server context based on a (encrypted)
private key.

Result:

It is now possible to create a JDK based SSL server context with an
encrypted (password protected) private key.
More brief somaxconn logging

- Consistent log message format
- Avoid unnecessary autoboxing when debug level is off
- Remove the duplication of somaxconn path
Clean-up d9cccccbb3344997e016e6a3603126ce65705c4d

- Revert irrelevant formatting changes
- Rename resource files
  - Add .pem
  - Remove 'netty' from names
Add more utility methods to check the availability of the epoll transport

Related issue: #2733

Motivation:

Unlike OpenSsl, Epoll lacks a couple useful availability checker
methods:

- ensureAvailability()
- unavailabilityCause()

Modifications:

Add missing methods

Result:

More ways to check the availability and to get the cause of
unavailability programatically.