Call setlocale("") , not "C", if we want to set the default one

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
sed: document where we are more liberal then GNU

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
sed: remove now-redundant backslash-newline handling

4b0bb9e0fd9ec06e9e61b1a1527ace99ea9fe571 added this code in 2007,
then in a2215b98f7d65bc613b9c8f008d79672402c6a07 more general fix
was added.

function                                             old     new   delta
add_cmd_block                                         98      58     -40

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

function                                             old     new   delta
tail_main                                           1548    1613     +65
tail_read                                            136      34    -102
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 1/1 up/down: 65/-102)           Total: -37 bytes

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