Implement --preserve-fds flag

[yihuaf]

Fix #145.

Note, the "LISTEN_FDS" and "LISTEN_PID" are not addressed in this PR. They are used for systemd and require us to also set the environment variables correctly. I or someone else will/should address these two in a different PR. This PR only addressed the --preserve-fds flag.

Test:
Using busy box, before --preserve-fds flag, a number of fds that should be closed.

/ # ls -ahl /proc/1/fd
total 0      
dr-x------    2 root     root           0 Aug  2 02:52 .
dr-xr-xr-x    9 root     root           0 Aug  2 02:52 ..
lrwx------    1 root     root          64 Aug  2 02:52 0 -> /4
lrwx------    1 root     root          64 Aug  2 02:52 1 -> /4
lrwx------    1 root     root          64 Aug  2 02:52 10 -> /dev/tty
lrwx------    1 root     root          64 Aug  2 02:52 13 -> /ptmx
lrwx------    1 root     root          64 Aug  2 02:52 14 -> /4
lrwx------    1 root     root          64 Aug  2 02:52 2 -> /4
lrwx------    1 root     root          64 Aug  2 02:52 3 -> socket:[1496697]
lr-x------    1 root     root          64 Aug  2 02:52 5 -> /

after, correctly only leaving 0,1,2 fds. 10 is tty opened for shell.

/ # ls -ahl /proc/1/fd
total 0      
dr-x------    2 root     root           0 Aug  2 02:53 .
dr-xr-xr-x    9 root     root           0 Aug  2 02:53 ..
lrwx------    1 root     root          64 Aug  2 02:53 0 -> /4
lrwx------    1 root     root          64 Aug  2 02:53 1 -> /4
lrwx------    1 root     root          64 Aug  2 02:53 10 -> /dev/tty
lrwx------    1 root     root          64 Aug  2 02:53 2 -> /4
/ # 

[Furisto]

@yihuaf Looks good to me. Just for my own understanding: Why does the tty stay open? If you close all file descriptors above 2, wouldn't it also be closed?

[yihuaf]

@yihuaf Looks good to me. Just for my own understanding: Why does the tty stay open? If you close all file descriptors above 2, wouldn't it also be closed?

I believe the tty is newly opened after we launch into a shell through docker, after the execvp.