Introduce automatic pager for read commands

Automatically show the output of show, policy, list, search, showsrc in a pager.

The pager setup is inspired by git's pager setup. Notably, the pager is found using APT_PAGER and PAGER variables.

We wait for the pager to be setup somewhat correctly by using a notify pipe to figure out whether execvp() was succesful - then the pipe will read EOF as the other end got closed by CLOEXEC during exec - or not, then the pipe will contain an errno.

We set up the correct handlers for signals and exit to close the fds and wait for the pager. Notably inside the signal handler we cannot flush our streams, only close them, so there is some duplication.

We call the InitOutputPager() function from inside the various Do...() functions rather than setting it up generally in InitOutput(). Doing so allows us to first render the progress without a pager, and then setup the pager for the content only which improves user experience.

When we setup a pager we also take care to disable standard input, as we should not be prompting users while a pager is running (the pager will be reading from the tty directly). We do this by dup2-ing() a /dev/null over it; if we just close()d the fd, another open() might reuse the fd number and problems could occur.