less: move "retry-on-EAGAIN" logic closer to read ops

This makes "G" (goto end of input) command work as well as
/search_for_nonexistent_string: both will read to EOF now
even from somewhat slow input (such as kernel's "git log").

function                                             old     new   delta
ndelay_on                                             35      43      +8
ndelay_off                                            35      43      +8
read_lines                                           695     691      -4
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 2/1 up/down: 16/-4)              Total: 12 bytes

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

function                                             old     new   delta
read_lines                                           715     695     -20

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