postinstall fixes bad ownership/permissions on LaunchDaemon

[mpanighetti]

• preinstall script only attempts to forget legacy package receipt if it is present on the system
• postinstall script sets LaunchDaemon ownership and permissions (in case files were modified prior to distribution and ownership/permissions were not properly set) #36
• added Troubleshooting section to README to address manual ownership/permissions repair on LaunchDaemon if package is not assembled with munkipkg
• fixed release links in CHANGELOG to point to current domain
• removed logger code from preinstall and postinstall (install.log can be used for installer diagnostic purposes in these cases)
• changed postinstall to POSIX Shell (Bash not necessary due to script simplicity)
• removed unused StartInterval attribute read

[mpanighetti]

Verified script successfully defers and subsequently installs security updates (tested in macOS Mojave VM running 10.14.6 (18G3020) with Security Update 2020-002 and Safari 13.1 pending).