installman only needs match =head1 to identify files with Pod.

installman only needs match =head1 to identify files with Pod.

Commit 63fae90782a9851d made installman skip files which contained no Pod.
However, the regex used, /^=(?:head\d+|item|pod)\b/ is overly permissive.
All files with Pod actually match /^=head1\b/. Moreover, a Pod file which
does not start with a =head1 is not going to generate a well formed man
page. Hence restrict the files processed to those which are well formed.