Add digest sum handling to the tool.

Android might want to replace the system *sum (i.e. md5sum, sha256sum
etc) binaries with a symlink to the BoringSSL tool binary.

This change also allows the tool to figure out what to do based on
argv[0] if it matches one of the known commands.

Change-Id: Ia4fc3cff45ce2ae623dae6786eea5d7ad127d44b
Reviewed-on: https://boringssl-review.googlesource.com/2940
Reviewed-by: Adam Langley <agl@google.com>