improve plpinfo page

improve plpinfo page

Fix version display in Perl >= 5.10: $^V no longer consists out of
single characters for each level.  Use the recommended practise of
formatting with sprintf (see perlvar) which seems to work in at least
5.6.1 (ie good enough).

Add 'Perl API' version analogous to 'PHP API' in phpinfo().
Some fallbacks if variables are empty or undefined.