Clarify internal function name
Register elarm_summary_sup invoking gproc in the OTP way `{via,_,_}`...

... rather than invoking it directly.  gproc is kept hardcoded in the
code - it is not in a configuration parameter.

Support for alternative registry was added in OTP behaviours by commit
b042673, that was first included in R15B01.

See also https://github.com/esl/elarm/issues/19
Let the user of elarm decide the way to register the elarm_summary_sup

Also use Opts argument in elarm:start_server/1.
Test registration of the elarm_summary_sup process