harden qnx runtime signal and prompt paths

[wordgitet]

Hello Brian,

I put together a another PR that hardens the QNX runtime path in a few targeted spots:

• trap.c: make signal message handling more defensive (sys_siglist bounds-safe + stable fallback)
• confstr.c: avoid overriding libc confstr() on QNX
• lex.c: harden PS1 hostname expansion (\h / \H) when gethostname() fails

I tested this with QNX SDP 8.0 in QEMU and the shell now starts cleanly, shows the expected hostname prompt, and handles basic commands/arithmetic correctly.

Quick demo from VM:

Startup complete
QNX oksh-smoke 8.0.0 2025/07/30-19:24:08EDT x86pc x86_64
 
# 
# 
# /system/bin/oksh
oksh-smoke# echo "
> Hi
> "

Hi

oksh-smoke# echo $((3+7))
10
oksh-smoke# 

Cross-builds also pass for:

• qcc -V12.2.0,gcc_ntox86_64
• qcc -V12.2.0,gcc_ntoaarch64le

Linux native build/smoke stayed fine on my side too.

Note: this PR focuses on runtime hardening. It does not yet add stronger configure-time signal detection for cross-build scenarios (--no-thanks), which I can experiment in a separate follow-up.