Command-line option to dump SHA1 digests of all source files.

Now, when you call tor --digests, it dumps the SHA1 digest of each
source file that Tor was built with.  We support both 'sha1sum' and
'openssl sha1'.  If the user is building from a tarball and they
haven't edited anything, they don't need any program that calculates
SHA1.  If they _have_ modified a file but they don't have a program to
calculate SHA1, we try to build so we do not output digests.