Const variables in prototypes don't add much value, but make the code
larger and redundant. Remove these const keywords.
Note, this doesn't apply to pointers to const memory.
@sjaeckel integrated clang-format with formal coding style. Run his
script and commit changes.
There are pros and cons of this commit.
Mixed coding style is a "broken window". A good single style simplifies
reading and writing code.
On the other hand, this is a big change which will lead to conflicts.
For example, XEP-0115 requires binary digest to be base64-encoded.
Add short function to simplify such a digest computation for a single
buffer. This function doesn't allocate memory. Therefore, it can't
fail and doesn't require strophe context (xmpp_ctx_t).
