Build: spec: fix redundantly carried /usr/share/pacemaker/api/ files

These would be delivered (co-owned in practice) with both -cli&-schemas
and since the former cannot exist without the latter (actually without
-libs, but in turn, -schemas present in the same transitive closure) and
since -cli doesn't make any direct references to the particular new API
schemas anyway (judging per "Use formatted output in stonith_admin"
commit), said files unambiguously belong to -schemas.

Based on Ken's input, we actually restructure the whole
/usr/share/pacemaker thing so that this directory, together with
explicitly picked contained files and directories, is likewise owned
by -schemas. This arrangement allows for rather loose downstream
additions to said location, like in case of nagios subdirectory
holding contents of sibling nagios-agents-metadata project (for
apparent reasons, we want the upstream spec file to be self-contained,
hence we omit such addition here).

Finally, there's a pre-existing packaging discrepancy highlighted, too

• we violate packaging policies of some downstreams in the sense that

we put PCMK-MIB.txt file into the directory that's not possessed by
any prerequisite. One possible solution for that would be to move that
file likewise into /usr/share/pacemaker hierarchy and let the users
copy the file from here should they make an active use of it, having
a dedicated -snmp package (requiring net-snmp properly) another.