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
Merge branch 'issue_2658' of github.com:akamai/netty into issue_2658
1. InboundHttp2ToHttpAdapter construction logical flaw corrected by introducing static construction. 2. Keep-Alive and Proxy-Connections added to HttpHeaders.Names as deprecated items. 3. Various other code clean up.