declare proc_dir earlier and resolve exact proc_dir

[Riolku]

We can declare the proc directory earlier in the code and use it. We intentionally add a trailing slash on line 352, in case for instance, the process PID is 123 and they request /proc/12345. Finally, we add a check for the literal directory /proc/getpid()

[dmoj-build]

Can one of the admins verify this patch?

[kiritofeng]

ok to test