sed: fix memory leak in 'r FILE' command

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
sed: document -E in help text

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
sed: fix "sed clusternewline" testcase

function                                             old     new   delta
process_files                                       2197    2226     +29
flush_append                                          47      54      +7
get_next_line                                        184     189      +5
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 3/0 up/down: 41/0)               Total: 41 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>