Fix kill pager pid throwing Errno::ESRCH when pager process already terminated

[tompng]

Fixes #847

In some case, when IRB receives IRB::Abort, pager process might be already terminated.
This can happen with timing or pager's problem.

Timing problem

When there is a sleep 1 just after pager.close, (press q to terminate pager and press \C-c within 1 sec) process might be already terminated. I can't reproduce this without inserting sleep 1 but it could potentially happen.

Pager's problem

Less somehow does not receive \C-c when paging content is large (more than 72KB) and has many lines.
Just after exiting pager by typing q, IRB receives the previous \C-c and IRB tries to Process.kill pid but the process is already terminated.

To reproduce:

irb(main):001> `less --version`
=> "less 581.2 (POSIX regular expressions) ...
irb(main):002> Object.new.tap{def _1.inspect; "a\n"*36*1024;end} # > To reproduce with macOS less 581.2
# Press \C-c and then q
lib/irb/pager.rb:36:in `kill': No such process (Errno::ESRCH)