net-fs/nfs-utils: warn if client tracking will be broken

net-fs/nfs-utils: warn if client tracking will be broken

NFSDv4 supports client tracking across server restarts using two
methods. The legacy method is in-kernel and requires CONFIG_CRYPTO_MD5,
which is not automatically selected by CONFIG_NFSD_V4 (see
https://bugzilla.kernel.org/show_bug.cgi?id=52271). The newer method
involves a usermode helper upcall program (nfsdcltrack), which is built
and installed only when USE="nfsdcld". If neither method is available,
then the NFS server does not support tracking clients across restarts,
and it emits a warning to the kernel log:

NFSD: unable to generate recoverydir name (-2).
NFSD: disabling legacy clientid tracking. Reboot recovery will not function correctly!

This commit introduces an ewarn in pkg_setup to warn the user if their
configuration will not support client tracking due to neither of these
options being enabled.

Closes: https://github.com/gentoo/gentoo/pull/1448
Closes: https://bugs.gentoo.org/582714
Signed-off-by: Matt Turner <mattst88@gentoo.org>