play: Convert com_jmp() to lopsub.

play: Convert com_jmp() to lopsub.

The command handler needs to check that there is a single argument
which is an integer between 0 and 100. This check needs to be modified
to use the lopsub primitives rather than accessing the argument
vector directly.

Unfortunately we can not perform the conversion of the string argument
into an int value with lopsub since the given percentage is an unnamed
option rather than an argument of an option.

The patch also rewords the summary and the help text of the jmp
subcommand slightly to be more clear.