Use SIGUSR1 rather than SIGTRAP for "allocated after fork"
Use SIGUSR1 rather than SIGTRAP for "allocated after fork"

Some platforma (eg ARM64) apparently generate SIGTRAP for panic abort!

See eg
  https://github.com/rust-lang/rust/pull/81858#issuecomment-840702765

This is probably a bug, but (i) we want to avoid that bug rather than
trying to fix it now and (ii) it would better to use a signal that is
less at risk of strangeness.

I grepped the rust-lang/rut codebase for SIGUSR and there were no hits.

Signed-off-by: Ian Jackson <d68d4287c1430ca128ad85033356c9d259238c74@chiark.greenend.org.uk>

aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust