vi: fix replacement of single character with CR

vi: fix replacement of single character with CR

Currently if the 'r' command is followed by a carriage return a
literal CR replaces the current character.

Fix this so that:

- a new line is inserted
- the autoindent setting is respected
- the cursor is placed at the start of the new line

function                                             old     new   delta
do_cmd                                              5052    5060      +8
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 1/0 up/down: 8/0)                 Total: 8 bytes

Signed-off-by: Ron Yorston <rmy@pobox.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>