Add security advisories affecting 14.3R, 14.4R and 15.0R

FreeBSD-SA-26:18.setcred affects all supported releases
FreeBSD-SA-26:19.file affects all supported releases
FreeBSD-SA-26:20.fusefs affects all supported releases
FreeBSD-SA-26:21.ptrace affects all supported releases
FreeBSD-SA-26:22.casper affects all supported releases
FreeBSD-SA-26:23.bsdinstall affects all supported releases
FreeBSD-SA-26:24.cap_net affects all supported releases