Add `:show linker` command to `:help` output

See http://ghc.haskell.org/trac/ghc/ticket/8074
Add `:show imports` to completion table

See http://ghc.haskell.org/trac/ghc/ticket/8075
Implement `%l` prompt substitution for showing current prompt line-number

See also http://ghc.haskell.org/trac/ghc/ticket/8047
Add the ability to customize the continuation prompt.

- Remove unused property `def_prompt`.

(cherry picked from ghc/ghc@bc44435dc2f6cda1071c68b79ace5b390a89244c)
Use prompt2 in `:set +m` mode

See also http://ghc.haskell.org/trac/ghc/ticket/8076
Fix multi-line input line-numbers & interaction between `:{` and `:set +m`

See http://ghc.haskell.org/trac/ghc/ticket/8051
Update `.cabal` for 7.6.3.3 release