Fix missing PF_NO_MANA in get_pflag_desc()

While I was at it, I also moved the descriptions to list-player-flags.h
to keep things DRY and to hopefully prevent similar issues in the future.
Fix Warrior birth screen showing "Learns  magic"
Merge pull request #374 from BardurArantsson/restruct-fix-missing-pflag

Fix missing PF_NO_MANA in get_pflag_desc()
Merge pull request #375 from BardurArantsson/fix-warrior-birth-learns-blank-magic

Fix Warrior birth screen showing "Learns  magic"