#2141 fix Kernel.system() exit status

[h7kayama]

This PR fixes #2141

The child process was different because which #{pager_executable} was execute on the first run.

Before

⋊> ~/g/g/h/pry on master ◦ bundle exec pry
[1] pry(main)> $?
=> #<Process::Status: pid 24738 exit 0>
[2] pry(main)> $?
=> #<Process::Status: pid 24778 exit 0>
[3] pry(main)> $?
=> #<Process::Status: pid 24778 exit 0>

I thought it would be better to execute which #{pager_executable} at the time of initialize.

After

⋊> ~/g/g/h/pry on fix_child_process  bundle exec pry
[1] pry(main)> $?
=> #<Process::Status: pid 24214 exit 0>
[2] pry(main)> $?
=> #<Process::Status: pid 24214 exit 0>
[3] pry(main)> $?
=> #<Process::Status: pid 24214 exit 0>
[4] pry(main)>