quirks: use basename in a POSIX compliant way

The POSIX version of basename modifies the string (and therefore crashes
on static strings), so use safe_strdup before calling it.

glibc provides a POSIX version when libgen.h is included.
FreeBSD 12 provides a POSIX version when nothing is included, which was
causing a segfault.

Using the POSIX version correctly is the right way to avoid any such issues.