[oxlog] add sigpipe::reset

[sunshowers]

We have a number of CLI tools that panic when piped to things like head
instead of quietly exiting. There's a long history about this within the Rust
community (see rust-lang/rust#62569), but the long
and short of it is that SIGPIPE really should be set to its default handler
(SIG_DFL, terminate the process) for CLI tools.

Because oxlog doesn't make any network requests, reset the SIGPIPE handler to
SIG_DFL.

I looked at also potentially doing this for some of our other CLI tools that
wait on network services. This should be fine to do if and only if whenever we
send data over a socket, the MSG_NOSIGNAL flag is set. (This causes an
EPIPE error to be returned, but no SIGPIPE signal to be generated.)

Rust does set this flag here 1. However, as of Rust 1.77 this flag is not
set on illumos. That's a bug and I'll fix it in Rust upstream.