Make smartparens brace smarter in C

By default, smartparens open brace would leave the brace at the wrong
indentation level.  Add a post handler for the open brace that
automatically indents the braces properly and leaves the cursor on an
open line in between the braces.  Example taken from the smartparens
wiki.
Use c common hook directly

The prelude-c-mode-common-hook doesn't seem to work correctly in the
presence of yasnippet.  Just use c-mode-common-hook directly to apply C
mode customization.