Add a pmpgpsig_t struct to the database entry struct and functions for
the lazy loading of database signatures. Add a function for checking
database signatures, reusing (and generalizing) the code currently used
for checking package signatures.
TODO: The code for reading in signature files from the filesystem is
duplicated for local packages and database and needs refactoring.
Signed-off-by: Allan McRae <allan@archlinux.org>