Still using BOOL where we need Win32 API compatibility.
Using JSBool instead of BOOL or bool where it matters.
Changed most relevant TRUE/FALSE to true/false too (though it's not as critical).

You shouldn't need to #include <stdbool.h> anywhere now - gen_defs.h should do that automatically/correctly based on the language/version/tool. In C23, stdbool.h isn't even needed for bool/true/false definitions (they're keywords), so we don't bother including stdbool.h in that case.
Microsoft didn't define __STDC_VERSION__ in their older tool chains (even though they were C99 compatible and had stdbool.h), so we use a _MSC_VER check to know that there's a stdbool.h we should use in that case.

For other/old compilers (e.g. Borland C) we #define bool/true/false following the pattern of stdbool.h (doesn't use a typedef).

I didn't convert UIFC yet.

This addresses issue #698

The default character set for outbound mail is now auto-determined (when not explicitly specified for a message) between UTF-8, ASCII, and CP437. The [mail] DefaultCharset setting (which fell-back to iso-8859-1 if blank) is no longer "a thing".

Also: specify 8-bit content-transfer-encoding for the (potentially UTF-8 or CP-437) plain text portion of a MIME-encoded message with file attachment (7-bit was wrong) and pass down the text sub-type (e.g. could be "html") for inclusion in the mime-part header (don't assume text/plain, but still use that as default).

Also: log an error when failing to delete an attached file (e.g. from data/file/*.out).

The "official" log line format is (as of right now at least):
"[socket] [protocol] [user/host-ID] [! if error]message"
Hopefully I can remember that when I add new log messages.

Also setting the protocol to SMTPS, POP3S, SEND/TLS, when appropriate and
passing it around to pretty much any function that can log a message.
Added debug-level log output when the mail sever is sending message body text
(every 100 lines).

format not a string literal and no format arguments [-Wformat-security]

Support STARTTLS in SMTP and STLS in POP3.  This includes the sendmail
thread.  Clients can now be configured to use STARTTLS with Synchronet
mail services, and Synchronet will attempt to deliver securely when
possible (though it will fall back to plain text delivery when necessary.)

This touches a *lot* of mail server stuff, so some instability is not
unlikely.

Of course, RFC8314 (Jan, 2018) officially recommends implicit TLS on the TLS
ports now, so support for that should be next.

charset" for the message plain-text instead of hard-coding ISO-8559-1. If no
default charset is configured, specifies ISO-8559-1 just as before.

all exported functions and data types using a 32-bit time_t (time32_t).
(part 2 of 2)

Changed mime blurb.

Changed order of arguments.