add available context info to debug log file headers

[whilei]

Since version, context, and session data is available from common, let's use it in debug log files to provide additional context for interpreting debug logs.

Changes this:

Log file created at: 2018/05/28 22:06:40
Running on machine: node-geth-500-k969c
Binary: Built with gc go1.9.4 for linux/amd64
Log line format: [IWEF]mmdd hh:mm:ss.uuuuuu threadid file:line] msg

To this:

Log file created at: 2018/05/29 07:15:11
Binary: Built with gc go1.9.2 for darwin/amd64
Context: VERSION=whilei.v5.3.0+22-58cea72[branch=feat/debug-log-with-version-header,commit=problem:-debug-log-files-lack-available-context-info,built=May/29Tue-07:14:48] GO=go1.9.2 GOOS=darwin GOARCH=amd64 SESSIONID=CAUt HOSTNAME=mh USER=ia MACHINE=a2c2f59f PID=18704
Log line format: [IWEF]mmdd hh:mm:ss.uuuuuu threadid file:line] msg