mport package fetch and clean paths are vulnerable to TOCTOU filesystem races

mport is the MidnightBSD Package Manager. Prior to 2.7.8, privileged package fetch and cache-cleaning operations used race-prone path handling across libmport/fetch.c, libmport/clean.c, libmport/util.c, libmport/bundle_read_install_pkg.c, libmport/delete_primative.c, and libexec/mport.create/mport.create.c. A local attacker with write access to a participating package cache or staging path could race path checks and replacement operations to redirect package downloads, cleanup, or install-related side effects outside the intended cache. The affected lifecycle helper paths also used shell-form invocation, increasing command-line interpretation risk during privileged helper execution. This issue is fixed in version 2.7.8.