nicer name-conflict restarts for common cases

  For IMPORT: offer option to SHADOWING-IMPORT the new symbol or skip
  importing.

  For EXPORT: offer option to KEEP-OLD or TAKE-NEW. (Shadowing or
  uninterning the other one.)

  For USE-PACKAGE: offer option to KEEP-OLD or TAKE-NEW. (Shadowing or
  uninterning the other ones.)
DEFPACKAGE at variance restarts

  Signal a full error on variances, in each case offering the user the
  choice of keeping the old stuff or dropping it.

  Fixes lp#891351.
remove deleted packages from implementation-packages list of other packages
update NEWS