debuginfod: Turn -d ":memory:" into -d "file::memory:?cache=shared"

debuginfod: Turn -d ":memory:" into -d "file::memory:?cache=shared"

debuginfod opens the database twice, once in read/wrote and once in
read-only mode. This means the magic ":memory:" in-memory database
cannot be used as is because the two connections don't really share
the underlying database. Fix this by turning ":memory:" into
":file::memory:?cache=shared" which makes the in-memory database
shared. See https://sqlite.org/inmemorydb.html

Document this in debuginfod.8 and make some tests use -d :memory:

Signed-off-by: Mark Wielaard <mark@klomp.org>