Add test for POD Spelling errors

This test is disabled by default.

To enable this test, you need to install the perl module Test::Spelling, and
set the environment variable TEST_POD_SPELLING.  You also need to have a
working spell checker Test::Spelling can use, preferably one which matches the
language of the text in the POD sections.
Fix spelling mistakes in POD sections

On the affected lines, leading and trailing whitespace has also been fixed.