Support single-part MIME-encoded messages in SMBLIB

This involved the removal of the content-transfer-decoding feature of the SMTP mail server since we need single-part MIME-attached file contents to be stored in their original encoded form (e.g. base64-encoded) and not in binary, for the message body text (where NULs aren't allowed, CR/LF is appended, etc).

I accidentally made this change to smbtxt.c in the new_file_base branch and then copied over here, so there's some unrelated innocuous changes (comment header, removal of SMBCALL) that hopefully won't cause a merge conflict later.

I don't actually receive single-part MIME attached files, so depending on others (e.g. Nelgin and Dream Master) to test for me.