MINOR: debug: report haproxy and operating system info in panic dumps

MINOR: debug: report haproxy and operating system info in panic dumps

The goal is to help figure the OS version (kernel and userland), any
virtualization/containers, and the haproxy version and build features.
Sometimes even reporters themselve can be mistaken about the running
version or environment. Also printing this at the top hepls draw a
visual delimitation between warnings and panic. Now we get something
like this:

  PANIC! Thread 1 is about to kill the process.

  HAProxy info:
    version: 3.3-dev3-c863c0-18
    features: +51DEGREES +ACCEPT4 +BACKTRACE -CLOSEFROM +CPU_AFFINITY (...)

  Operating system info:
    virtual machine: no
    container: no
    kernel: Linux 6.1.131 #1 SMP PREEMPT_DYNAMIC Fri Mar 14 01:04:55 CET 2025 x86_64
    userland: Slackware 15.0 x86_64

  * Thread 1 : id=0x7f615a8775c0 act=1 glob=0 wq=1 rq=0 tl=0 tlsz=0 rqsz=0
        1/1    stuck=0 prof=0 harmless=0 isolated=0
               cpu_ns: poll=1835010197 now=1835066102 diff=55905
               (...)