Optimize evaluation of environment variables
Optimize evaluation of environment variables

This patch addresses the following issues:

* When RELX_REPLACE_OS_VARS is set, evaluation of environment vars
  is done by the shell rather than awk, this allows to use more
  powerful notation of environment variables in sys.config and vm.args
  (e.g. `-sname abc@${HOSTNAME,,}` or `{myapp, [{user, ${USER:-unknown}}]}`
* Using shell vars rather than unnecessarily forking awk/grep/etc

tsloughter/relx,ingwinlu/relx,erlware/relx,lrascao/relx