rxvt-unicode (and all children) ignore SIGFPE

[Anders Kaseorg]

My friend just ran across a problem where rxvt-unicode ignores SIGFPE, and 
its child processes inherit this state, leading to undefined behavior.  
The signal is ignored inside PERL_SYS_INIT3:

https://rt.perl.org/rt3/Public/Bug/Display.html?id=114574

This might eventually get fixed in Perl upstream, but it seems dangerous 
enough that it’s probably worth adding a workaround in rxvt-unicode.

Anders