MSI: Set REINSTALLMODE if not set to always

* src/make-msi.pl (product): Set REINSTALLMODE if not set.

We always want our REINSTALLMODE to be amus, so to replace all files.
But maybe some SCM wants to set this manually and hardcoding it creates
a warning. So we only set it if it is not already set.

For documentation what amus is vs. the default omus see:
https://learn.microsoft.com/en-us/windows/win32/msi/reinstallmode

a: Always replace everything.
o: Reinstall if the file is missing or is an older version.
It seems to be a common problem that with "o" files are uninstalled
during an upgrade but not actually reinstalled.

• GnuPG-Bug-Id: T6567