Conditionally omit `TemplateHaskell` from set of supported extensions

This emulates what GHC>=8 will do on its own for older GHC versions.

This is particularly useful in combination with #2644 allowing the
solver to toggle flags depending on the availability of
`-XTemplateHaskell`.

For instance, cross-compilers and/or unregisterised GHC builds often
don't have TemplateHaskell support. Having support for toggling flags
based on availability of `-XTemplateHaskell` allows `cabal` to
support such environments with less manual intervention.

Teach Cabal about GHC's Strict language extension

Use `-fno-warn-implicit-prelude' in Paths_* module

When GHC is invoked with `-fwarn-implicit-prelude', then autogenerated
Paths_* modules are source of unnecessary warnings.

This commit adds OPTIONS_GHC pragma with `-fno-warn-implicit-prelude' in
to Paths_* module, so that such warnings are suppressed.

Some more IBM AIX fixes

Otherwise this emits linker flags not suitable for XCOFF

IBM AIX requires its own proprietary linker which does not support
`--strip-unneeded`.

The `Permissive` classification is for recognising tokens from the
"outside world". `platformFromTriple` is similar to
`buildOS`/`buildArch`.

Specifically, on IBM AIX, the platform triple is usually
"powerpc-ibm-aix", where "powerpc" will only be recognised in
`Permissive` mode.

Disable Ar.wipeMetadata for AIX

AIX uses its own `ar` format variant (with a different header),
which causes Cabal to abort with

  Distribution.Simple.Program.Ar.wipeMetadata: Bad global header

This patch simply disables this non-essential operation for AIX.

Global Constraints Option

Make `cabal init` reject extra arguments.

binary-0.8.0.0 will provide Binary Version

This is better than ignoring them because one is likely in some state of
confusion when passing extraneous arguments to `cabal init` and would
probably appreciate being taken out of that state sooner rather than
later.

Show details direct

This mode implements #2911, and allows to connect the test runner
directly to stdout/stdin. This is more reliable in the presence of no
threading, i.e. a work-arond for #2398.

I make the test suite use this, so that it passes again, despite
printing lots of stuff. Once #2398 is fixed properly, the test suite
should probably be extended to test all the various --show-details
modes.

Update Travis job to use GHC 7.10.3

Implement `cabal upload --doc`

The test case is changed to print a rather large amount of text, large
enough to fill a buffer. This way, the buffer draining functionality in
the test runner is tested, and it reveals #2398.

This makes the test suite currently fail, of course, but the bug was
there before.

This gives newcomers like me a better chance to understand how the Cabal
test suite is constructed.

The powershell script contains the password, so writing it to a file
is a bit unsafe. We know pass it via stdin.
This commit also improves the error message for network failures a
little bit.