vi: fixes to string search in colon commands, closes 10321

vi: fixes to string search in colon commands, closes 10321

Handling of string searches in colon commands (e.g ':/pat1/,/pat2/cmd')
differ from standard vi:

- As reported in bug 10321 such searches can't be repeated using the
  'n' command.  This is because the last search pattern isn't updated.

- The search also can't be repeated using the command '://' because
  an empty search pattern doesn't imply the use of the last search
  pattern.

- Such searches should start on the line after the current line,
  otherwise '://' never moves to the next occurrence of the pattern.
  This can also affect other cases where line ranges are specified
  using search patterns.

Fix these various issues.

function                                             old     new   delta
get_one_address                                      325     342     +17

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