webhttrack: drop Linux-only SIGSTKFLT from the cleanup trap

SIGSTKFLT does not exist on macOS/BSD, so bash rejects the trap command there
and the signal cleanup may fail to install. Interrupting webhttrack (Ctrl-C)
could then leave htsserver running and the temp server file behind. Drop
STKFLT; the remaining signals are all POSIX.

Closes #545

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Signed-off-by: Xavier Roche <roche@httrack.com>

src/webhttrack

@@ -132,12 +132,12 @@ function cleanup {
 }
 
 # Cleanup in case of emergency
-trap "cleanup now; exit" HUP INT QUIT ILL TRAP ABRT BUS FPE SEGV PIPE ALRM TERM STKFLT XCPU XFSZ
+trap "cleanup now; exit" HUP INT QUIT ILL TRAP ABRT BUS FPE SEGV PIPE ALRM TERM XCPU XFSZ
 
 # Got SRVURL, launch browser
 launch_browser "${BROWSEREXE}" "${SRVURL}"
 
 # That's all, folks!
-trap "" HUP INT QUIT ILL TRAP ABRT BUS FPE SEGV PIPE ALRM TERM STKFLT XCPU XFSZ
+trap "" HUP INT QUIT ILL TRAP ABRT BUS FPE SEGV PIPE ALRM TERM XCPU XFSZ
 cleanup
 exit 0