Load decoders from all search paths, not just the default.

Load decoders from all search paths, not just the default.

srd_decoder_load_all() was really only getting a list of decoders
from the default (installation) path, so could not find uninstalled
decoders, or those in a custom added search path.

This broke development of new PDs when using the SIGROKDECODE_DIR
environment variable, and broke many of the unit tests in the tests/
directory -- those actually tested against the already-installed
decoders, not the ones about to be installed.