Update the pager automatically when the window height changes

[Bruno Haible]

When you do
  .set pager yes
  .help
then reduce or increase the terminal window height, the pager outputs
too many / too few lines per page. To fix this, one needs a SIGWINCH
handler. This proposed patch does it: it causes the pager to "know"
about window size changes.

libreadline also has a SIGWINCH handler, but it knows how to not
interfere with the main program's SIGWINCH handler (cf. rl_catch_sigwinch
in the readline sources).

0003-Update-the-pager-automatically-when-the-window-heigh.patch
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