inspection: Fix inspection of recent RPM guests using non-BDB.

Recent RPM-based guests have switched from using Berkeley DB (BDB) to
sqlite.  In order to inspect these guests (and earlier ones) we need
to stop using the hokey parsing of the BDB and use librpm APIs
instead.

This commit adds a new internal API so we can call librpm from the
daemon, and changes the library part to use the new API for RPM-based
guests.

This change removes the requirement for BDB tools like db_dump.

See also:
http://lists.rpm.org/pipermail/rpm-ecosystem/2021-March/000751.html
http://lists.rpm.org/pipermail/rpm-ecosystem/2021-March/000754.html
https://blog.fpmurphy.com/2011/08/programmatically-retrieve-rpm-package-details.html

This breaks the virt-inspector test (now in the separate guestfs-tools
repository).  However this is not a bug in libguestfs, but a bug in
the phoney Fedora guest that we use for testing - we created a
BDB-style RPM database which was supposed to be just enough to make
the old code work.  The new code using real librpm needs
/usr/lib/rpm/rpmrc (not present in the phoney image) and also cannot
parse the phoney database, so we will need to separately rework that
test.

Thanks: Panu Matilainen
Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1766487
Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1409024

docs/guestfs-building.pod

@@ -210,11 +210,6 @@ eg. F</etc/libguestfs-tools.conf>.
 Optional.  Used by the L<libvirt backend|guestfs(3)/BACKEND> to
 securely confine the appliance (sVirt).
 
-=item Berkeley DB utils (db_dump, db_load, etc)
-
-Optional.  Usually found in a package called C<db-utils>,
-C<db4-utils>, C<db4.X-utils> etc.
-
 =item systemtap
 
 Optional.  For userspace probes.
@@ -252,6 +247,10 @@ Optional.  Render icons from guests.
 
 Optional.  Render icons from Windows guests.
 
+=item librpm
+
+Optional.  To parse the list of applications from RPM-based guests.
+
 =item Perl C<Expect>
 
 Optional.  Perl module used to test L<virt-rescue(1)>.