Reload SSL certificates on signal SIGHUP

Catch SIGHUP, use it to reload configuration (SSL certs), similar to
nginx and other server programs.  This allows easy automation of
reloading certificates, an important factor with services such as
Let's Encrypt.

If reloading certificates fails, the old certificates are kept to
avoid disrupting new connections until the situation is sorted out.

Resolves GH-208.

(cherry picked from commit 25a3ae50ac0d9835283e4f5f10fcfcc10ed5575d)