Support '--<option>=<value>' form for long options

The Linux version of inotifywait uses getopts to parse the commandline
arguments. This supports long options in two formats: the regular
"argument" '--<option> <value>' form, and the "equals" '--<option>=<value>'
form.

To improve portability of scripts that might use the "equals" form, add
support for this to the argument parser.

Usage of getopts in inotifywait: https://github.com/rvoicilas/inotify-tools/blob/master/src/inotifywait.c#L560
Man page of getopts which also describes the '--<option>=<value>' form: http://linux.die.net/man/3/getopt