Ensure that "mail from" works on Debian

[wscheicher]

Describe the bug

mail-service-notification.sh guesses based on the distribution which mail command is used.

For Debian mailutils is assumed, but without any verification.
See mail-service-notification.sh L180

To Reproduce

On a debian system, install exim4 (which by default installs bsd-mailx via recommends), install icinga2, and try to specify "mail from"

Your Environment

Icinga2: 2.14.5-1+debian12 from packages.icinga.com
Debian Bookworm

Additional context

Maybe it would be a idea to check readlink -f /etc/alternatives/mail on debian to check if it is mailx or mailutils?
Or see if mail -V returns a error (mailx) or version information and no error (mailutils)