/proc/[pid]/status is missing the signal mask lines

runsc leaves SigPnd, ShdPnd, SigBlk, SigIgn and SigCgt out of /proc/[pid]/status. The file jumps from Threads straight to CapInh, so nothing reading procfs can tell whether a process handles a given signal. Linux prints all five between those two lines, in fs/proc/array.c task_sig().

Grep finds no SigCgt line:

```
$ runsc do /bin/grep -c ^SigCgt /proc/self/status
0
```

Proposed fix in #14499.

Start by locating gVisor's procfs implementation for /proc/[pid]/status and compare its output with Linux's task_sig() in fs/proc/array.c. Run the reported runsc grep command and verify that SigPnd, ShdPnd, SigBlk, SigIgn, and SigCgt appear between Threads and CapInh.

Written by the indexing model from the issue text.