bin/elasticsearch: add help, fix endless loop

This change adds command line help for all options to the es start script.
Both `-h` and `--help` options are accepted.

Also, an endless busy loop in the long options parser was fixed: running the
script with a long opt parameter w/o value (e.g. `elasticsearch --buuuurrrnn`)
the long option parser would end up in an endless busy loop.

Fixes #2168

Signed-off-by: Thilo Fromm <github@thilo-fromm.de>