Change dsc verification to fail only if the public key was available, but
signature verification failed. If no public key is available for the dsc,
print warning only. (LP: #1700846)
Also add pull-* parameter --no-verify-signature to manually prevent failure
when signature verification fails.
rename only with no content changes, so next commit shows code
changes converting script pull-pkg to module pullpkg.py
This also changes the file mode from 755 to 644, since we're changing
an executable script into a module.