new: dev: Print OS platform in "named -V"

The "running on" line emitted by `named -V` (as well as the startup
log and `rndc status`, which share the same source) now appends the
PRETTY_NAME value from /etc/os-release in parentheses after the uname
output, e.g.:

    running on Linux x86_64 6.19.14-... (Fedora Linux 42 (Workstation Edition))

This helps disambiguate environments where the kernel string is not a
reliable indicator of the userspace, such as RHEL clones and
containers whose kernel does not match the host OS.

When /etc/os-release is absent, /usr/lib/os-release is tried as a
fallback per the systemd os-release(5) specification. When neither is
available or no PRETTY_NAME is found, the output is unchanged.

Assisted-by: Claude:claude-opus-4-7
Closes #5334

Merge branch '5334-add-os-platform-to-named-V' into 'main'

See merge request isc-projects/bind9!12055