cxxll::elf_image::dwarf_cu_range: New class
checksec: Add DWARF producer extraction
SQL schema: Minor reordering
Add TODO item: pg_exception in putCopyEndError()
cxxll::pg_query: Support const std::string * arguments in dispatching
cxxll::pg_put_copy::finish(): Actually reset conn_, even with an exception
cxxll::pg_put_copy::abort(): New member function
SQL schema: Add table symboldb.dwarf_compilation_unit

And supporting table symboldb.dwarf_producer and function
symboldb.intern_dwarf_producer.
database: Load DWARF compilation unit data
Add DWARF compilation unit integration test
cxxll::elf_image::dwarf_cu_range: Ignore some errors

These occur in practice.  We need a better way to report these failures.
database::add_dwarf(): Simplify

We make two passes over the DWARF compilation units and use the first
pass to collect all producer IDs.  This avoids the complicated rollback
procedure.