Fix comment for EPKG_ENODB
If opening the db in READ_ONLY give a warning if it is missing

This fixes no output for pkg-rquery and pkg-clean when there
is no database.
Give advice on how to recreate the database if auto-updating was skipped

New output:

  # pkg rquery %o git
  pkg: Repository FreeBSD has incompatible checksum format, need to re-create database
  pkg: Repository FreeBSD cannot be opened. 'pkg update' required
  # pkg clean
  pkg: Repository FreeBSD has incompatible checksum format, need to re-create database
  pkg: Repository FreeBSD cannot be opened. 'pkg update' required
  Nothing to do.
  # pkg upgrade -U
  pkg: Repository FreeBSD has incompatible checksum format, need to re-create database
  pkg: Repository FreeBSD cannot be opened. 'pkg update' required
  Checking for upgrades (0 candidates): 100%
  Checking integrity... done (0 conflicting)
  # pkg upgrade
  Updating repository catalogue
  pkg: Repository FreeBSD has incompatible checksum format, need to re-create database
  Fetching meta.txz: 100% of 944 B
  Fetching digests.txz: 100% of 2 MB
  Fetching packagesite.txz: 100% of 5 MB
  [...]