include/opt: Add flag to define required arguments

include/opt: Add flag to define required arguments

The opt module currently does not provide a means to define required
arguments, making it hard for the callee to determine if an argument
was provided on the commandline, or if the default value was returned
by opt_get().
This commit adds a flag to specify if an argument must be provided on
the commandline. Further, since multiple flags can now be passed to
opt_add_arg(), the flag format is changed: now, a string containing
all flags as characters must be passed. The implementation understands
the following flags:

 r - Argument is required
 v - Argument has a value (e.g. is followed by a second argument)

For example, to declare a required argument with a value, one would
pass the flags "rv".