fix: some security stuff

[ariasmn]

Some security stuff over the C code. I found two issues myself and got curious and asked DeepSeek V4 Pro for more input.

What I found was the PID check without upper bound which caused truncation. I then checked and was also present in the GUI obviously. What DeepSeek found (alongside a couple of false positives):

• Terminal escape injection: process names come from /proc/pid/comm, which any process can set to arbitrary bytes via prctl(PR_SET_NAME). pidnl list printed them raw, so a process could inject ANSI/OSC sequences into the admin's terminal (same class as CVE-2023-4016 in ps). Control characters are now stripped when reading the name.
• Protocol injection via executable paths: Linux filenames can contain newlines, and a process launched from such a path would desync the GUI's line-based protocol and misalign PIDs and limits. The same sanitization is applied to exe_path. (I actually tested myself and I couldn't believe lol)
• Monitor socket directory: the monitor daemon used to trust an existing /run/pidnl without checking what it was: stat follows symlinks, and there was no ownership/type check before unlink/bind on the socket. Now it requires a real root-owned directory via lstat, or fails hard.