Adjust newer Botan versions, further centralize Botan version
specific code.
* src/botan_glue.cc: add and move a PKCS8 key loader method, including the
necessary Dummy_UI class and special exception handling here. Introduce
an initialize_botan method, used by monotone.cc and unit tests as well as
a central load_pkcs8_key one.
* src/automate.cc: Eliminate a compiler warning.
* src/{botan_pipe_cache,gzip,hmac,key_store,lazy_rng,packet}.hh,
src/{database,file_io,gzip,hmac,inodeprint,key_packet,key_store,
merkle_tree,monotone,packet,sha1,ssh_agent,transforms}.cc,
test/unit/{tests/packet,unit_tester}.cc: Drop inclusion of "botan.h", as
it got deprecated with Botan 2.3.
* src/{key_store,packet}.cc: factor out loading of unprotected keys to the
new load_pkcs8_key method in botan_glue.cc.
* src/monotone.cc,test/unit/unit_tester.cc: factor out Botan initialization
to the new method initialize_botan in botan_glue.cc
* src/database.cc: Use non-deprecated constructor for PK_Exncryptor_EME.
* src/lazy_rng.hh: Use non-deprecated AutoSeed_RNG for newer Botan versions.
* src/botan.hh: rename back to botan_glue.hh, move some method definitions
here.
* Makefile.am: add botan_glue.{cc,hh}