lib/pager: fix output truncation from buffered stdout

lib/pager: fix output truncation from buffered stdout

Commit 518a0ad dropped fflush(stdout) and fflush(stderr) from
wait_for_pager() before STDOUT_FILENO and STDERR_FILENO are closed
because it is called from a signal handler and they are async-unsafe.

However, failure to flush output leads to unwanted truncation when
stdout is fully buffered, which it is by default when we are linked
against musl.

Ensure stdout and stderr are line-buffered before redirecting them to
the pager subprocess to avoid this.

Addresses: https://github.com/util-linux/util-linux/issues/2218
Signed-off-by: Chris Webb <chris@arachsys.com>