mport trusts environment-controlled temporary directories in privileged metadata extraction

mport is the MidnightBSD Package Manager. mport before 2.7.8 used TMPDIR while extracting package metafiles, including when running as root or in setuid/setgid contexts. An attacker able to control the environment for a privileged mport invocation could redirect temporary metadata extraction to an attacker-controlled location. PR 123 ignores unsafe TMPDIR values in privileged contexts and rejects empty TMPDIR. This issue has been patched in version 2.7.8.