small updates to Makefile.PL

Implemented a few suggestions from "Kwalitee": removed executable bit and
specified minimum Perl version. The lowest Perl version I've tested with is
v5.8.9, but Perl::MinimumVersion says I should be ok (at least syntax-wise)
down to v5.6.0.

Also I've specified the minimum ExtUtils::MakeMaker version so it's included
in the metadata.
turned Synopsis into a quick reference

Hopefully it will be more helpful this way.
simplified stdin/out/err option example code
enforced max line width for POD code blocks
a few general documentation updates
bring code coverage back up to 100%

Tickle IPC::Run3::Shell::_cmd2str() a bit more. (undef wasn't getting
tested anymore, probably due to 339991ba653bff)
updated Changes for v0.51