sed: open input files sequentially to avoid EMFILE

sed: open input files sequentially to avoid EMFILE

Currently, sed pre-opens all files, which may cause EMFILE errors
on systems with low ulimit -n.  Change sed to open one file at a time.

function                                             old     new   delta
get_next_line                                        177     235     +58
sed_main                                             682     652     -30
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 1/1 up/down: 58/-30)             Total: 28 bytes

Based on the patch by Daniel Borca <dborca@yahoo.com>

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