Add ability to set "Listen" option from environment variable

[tomsommer]

Based on #52 this adds the ability to set the option from an environment variable called SRS_LISTEN

[roehling]

Thanks for the pull request! Could you please change the following:

• rename SRS_LISTEN to SRS_LISTEN_ADDR to make it clearer what the parameter is supposed to do.
• put the -l option somewhere before the -X on the command line, because we had issues with the optional argument for -X being misaligned in the past.

[tomsommer]

Done