Add missing nolegacy/crypto.c and prf.c.
Fix src Makefile.

Commit cfe9285adf391ab66faeb5def811fe08e47a221a broke the build - elif
is not valid GNU Makefile syntax:

make[2]: Entering directory 'tinc/src'
Makefile:1135: *** missing separator (did you mean TAB instead of 8 spaces?).  Stop.

The correct syntax is "else <condition>", but unfortunately, automake
doesn't seem to like it very much:

src/Makefile.am:206: error: else reminder (GCRYPT_TRUE) incompatible with current conditional: OPENSSL_FALSE

Consequently, this commit uses a nested else-if instead. Annotations are
added to prevent confusion; these annotations are enforced by automake.
Fix unmatched parenthesis in tincctl.c.

Commit cfe9285adf391ab66faeb5def811fe08e47a221a broke the build:

tincctl.c:1852:3: error: expected ')'
                return 1;
                ^
tincctl.c:1851:4: note: to match this '('
        if(!rsa_keygen(argc > 1 ? atoi(argv[1]) : 2048, true)
          ^
Fix some char -> unsigned char implicit conversion warnings.

These were introduced by commit
cfe9285adf391ab66faeb5def811fe08e47a221a. sha512() expects unsigned
chars (for mathematical reasons, I presume), while the rest of the code
uses signed chars.
Fix typo in logging statement.

This was introduced in cfe9285adf391ab66faeb5def811fe08e47a221a.