Improve coredump_dir on FreeBSD and Solaris based OS (#974)

Improve coredump_dir on FreeBSD and Solaris based OS (#974)

Disclose that coredump_dir may also make the process "traceable",
including enabling core dumps and ptrace(2) attachments in certain
environments.

Add support for making the process traceable in FreeBSD- and
Solaris-like environments.

When Squid does not know how to make the process traceable in a given
environment, it does nothing under the assumption that the process is
already traceable enough (by default) to dump cores. Alternatively, we
could warn the admin about the lack of tracing support for that
environment. It is not clear which option is better, but we are betting
on the processes being traceable by default in most not explicitly
covered environments.