Workaround for what looks like an OS X-specific libedit issue

Other platforms may benefit from something similar if issues arise.  The
libedit library doesn't explicitly initialize the curses termcap library,
which it gets away with until TERM is set to VT100 where it stumbles over
an implementation assumption that may not exist on other platforms.

<rdar://problem/17581929>

llvm-svn: 226891