perldiag: miscellaneous clean-up

perldiag: miscellaneous clean-up

•‘Corrupted regexp opcode’ is a ‘can’t happen’ error, so it belongs
  in the P category.
• Two spaces after dots for consistency
• Rewrap for slightly better splain output
• The description usually begins on the same line as the category, so
  do so consistently
• Reorder alphabetically
• Missing category
• Single, not double, backslash
• Squash two adjacent (due to reordering) entries with identical
  descriptions
• ‘given’ does not depend on lexical $_ any more
• Remove duplicate entries (and placate diag.t with diag_listed_as)