Format truncation into the header variable in opendmarc.c

[bicknell]

On the develop branch, In opendmarc.c twice the "header" variable experiences format truncation:

Instance 1

opendmarc.c:3625:3: note: ‘snprintf’ output 32 or more bytes (assuming 2080) into a destination of size 1025
   snprintf(header, sizeof header,
   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
            "%s%s%s; dmarc=%s (p=%s dis=%s) header.from=%s",
            ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
            authservid,
            ~~~~~~~~~~~
            conf->conf_authservidwithjobid ? "/" : "",
            ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
            conf->conf_authservidwithjobid ? dfc->mctx_jobid : "",
            ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
            aresult, apolicy, adisposition, dfc->mctx_fromdomain);
            ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Instance 2

opendmarc.c:3081:3: note: ‘snprintf’ output 31 or more bytes (assuming 2079) into a destination of size 1025
   snprintf(header, sizeof header,
   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
            "%s; dmarc=permerror header.from=%s",
            ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
            authservid, dfc->mctx_fromdomain);

It appears to be used as a temporary variable for putting a new header in before adding it to the output. It is defined about line 2260:

        unsigned char header[MAXHEADER + 1];

The MAXHEADER length is defined in opendmarc.h:

#define MAXHEADER       1024

That define is used exactly one other spot, opendmarc-ar.c about line 714:

    u_char tmp[MAXHEADER + 2];

Note inconsistency with the +1 or +2.

Now, RFC2822 says:

   There are two limits that this standard places on the number of
   characters in a line. Each line of characters MUST be no more than
   998 characters, and SHOULD be no more than 78 characters, excluding
   the CRLF.

But also:

   Each header field is logically a single line of characters comprising
   the field name, the colon, and the field body.  For convenience
   however, and to deal with the 998/78 character limitations per line,
   the field body portion of a header field can be split into a multiple
   line representation; this is called "folding". 

Ergo, what I think needs to happen here is:

1. The buffer needs to be made larger, probably on the order of 2048 characters.
2. OpenDMARC needs to implement header folding if the output exceeds 998 characters.

[mskucherawy]

This is pretty minor, since all of the things that go into the header field have large-ish buffers but are almost always relatively tiny. Punting out at least to the next release.