signal: remove SIGURG from default-ignore list

[badoriie]

Problem

SIGURG is currently in iSH's default-ignore list in kernel/signal.c:

case SIGURG_: case SIGCONT_: case SIGCHLD_:
case SIGIO_: case SIGWINCH_:
    return SIGNAL_IGNORE;

Go 1.14+ uses SIGURG to implement asynchronous goroutine preemption (runtime/signal_unix.go). When a Go program calls signal.Notify or the runtime installs its own SIGURG handler via runtime_sigInstall, iSH's default-ignore classification prevents proper handler registration.

The result is a hard crash on startup:

fatal: bad g in signal handler

The workaround is to set GODEBUG=asyncpreemptoff=1 before launch, which disables Go's async preemption entirely — a significant runtime regression.

Fix

Remove SIGURG_ from the ignore list. Its POSIX default disposition is already "ignored" for processes that haven't installed a handler, so removing it from this list has no effect on programs that don't use SIGURG. It only allows programs like Go to successfully install their own handler.

// Before
case SIGURG_: case SIGCONT_: case SIGCHLD_:

// After
case SIGCONT_: case SIGCHLD_:

Impact

• Go 1.14+ programs start without GODEBUG=asyncpreemptoff=1
• No behavioural change for programs that don't use SIGURG
• Tested with Conduit on iSH (Alpine Linux, iPhone)