add new stub perldelta for 5.18.3
correct a Pod warning in the new perldelta template
Crash in tab completion with Term::ReadLine::Gnu.

Perhaps it also affects Term::ReadLine::Perl / Term::ReadLine::Perl5 .
I still need to test with PadWalker installed. No tests were added, but
it passes all existing tests.

Conflicts:
	lib/perl5db.pl
perldelta for 538acd8d5b