krb5: Move some manpages to other packages

Move KDC manpages to kerberos-kdc-man.

Move the generic Kerberos manpages (e.g., kerberos.7) to kerberos-man
instead of kerberos-lib-man. Although they technically describe
behaviour implemented in the libraries, conceptually, they are more
associated with Kerberos in general.

While here, remove some redundant SUFFIXES.

Reviewed by: manu, cy
Differential Revision: https://reviews.freebsd.org/D51515