The eln macro requires ELN SIG approval, and there is no need to differ
from RHEL here. More importantly, we don't want the dynamic
buildrequires in ELN either.
RHEL/ELN cargo macros do not support -f, so import the RHEL/c9s patch
which adds rdcore to the default features.
While we're at it, use the RHEL %%cargo_prep -V to handle vendoring
rather than manually modifing the cargo config.
