api: add nvim_create_buf to create a new empty buffer.

Loading existing files into a buffer is non-trivial and requires a window.
Creating an unnamed emtpy buffer is trivial and safe though, thus worth a
special case.

Change nvim_buf_set_option to use aucmd_prepbuf. This is necessary
to allow some options to be set on a not yet displayed buffer, such
as 'buftype' option.

vim-patch:7.4.1858: Add BLN_NEW to enforce buflist_new creating new buffer

src/nvim/buffer.h

@@ -31,7 +31,7 @@ enum bln_values {
   BLN_CURBUF = 1,   // May re-use curbuf for new buffer
   BLN_LISTED = 2,   // Put new buffer in buffer list
   BLN_DUMMY  = 4,   // Allocating dummy buffer
-  // TODO(mhinz): merge patch that introduces BLN_NEW
+  BLN_NEW    = 8,   // create a new buffer
   BLN_NOOPT  = 16,  // Don't copy options to existing buffer
 };