1. correct determination of width of left help column; make leading indent

  * A bug resulted in taking the width of a stringified array reference,
    rather than the data in the array.

  * The leading indent was a full tab, which is a lot of space. Reduce
    it to 4 spaces.

2. The help message didn't specify that an option took a value.

Here's before and after for the option specification in t/11-usage.t

Usage: 11-usage.t [-rv] [long options] [arguments]
	--bare
	--define                 (default: arch=i386, isize=4)
	--input                  (default: test.txt)
	--libs                   (default: one, two)
	-r                       recursive
	--test                   run in test mode
	-v, --verbose            turn on verbose output (default: 2)

Usage: 11-usage.t [-rv] [long options] [arguments]
    --bare <value>
    --define key=<value>                 (default: arch=i386, isize=4)
    --input <value>                      (default: test.txt)
    --libs <value>                       (default: one, two)
    -r                                   recursive
    --test                               run in test mode
    -v [<value>], --verbose [<value>]    turn on verbose output (default: 2)