add required headers for smtpd.h and remove unnecessary ones in other files.

ok jung@

Remove unused variables found by clang. Additional unused var spotted by eric@.

OK mvs@, eric@

When system calls indicate an error they return -1, not some arbitrary
value < 0. errno is only updated in this case. Change all (most?)
callers of syscalls to follow this better, and let's see if

When system calls indicate an error they return -1, not some arbitrary
value < 0. errno is only updated in this case. Change all (most?)
callers of syscalls to follow this better, and let's see if this strictness
helps us in the future.

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switch from having automatic EVP_CIPHER_CTX variables to allocating them
with EVP_CIPHER_CTX_new() and releasing them with EVP_CIPHER_CTX_free().

ok sunil@ and millert@

switch from EVP_{En,De}cryptInit & EVP_{En,De}cryptFinal to their _ex
counterparts, as suggested by bcook

ok bcook@, sunil@, eric@

remove spaces after '!'

no binary change

ok millert

bcopy -> memmove
bzero -> memset

fix bogus warning due to limited range on 32bit archs.

unused variable

Add crypto.c to provide smtpd with a way to encrypt envelopes and messages
using aes-256-gcm before they hit the queue. not "plugged" yet.

lots of comments from mikeb, tedu and djm.