Increase OPENDMARC_ARCSEAL_MAX_TOKEN_LEN to 768

[abokth]

Enough for 3072 bit key signatures. Should fix #183.

[thegushi]

Thank you for this fix — bumping the token length constant was the right instinct and this PR helped identify the root cause.

Two gaps remained that PR #296 addresses:

1. The same limit (OPENDMARC_ARCSEAL_MAX_LONG_VALUE_LEN) also exists in opendmarc-arcseal.h and controls the size of the signature_value field in struct arcseal. Bumping only the .c constant left the struct field at 512 bytes, so values from large-key signatures would still be silently truncated. PR Fix ARC header parsing crashes: SIGABRT on malformed tokens and SIGSEGV on large RSA key signatures #296 bumps both constants, and raises the limit to 2048 (covering RSA 4096-bit and larger).

2. A second, independent crash path existed where a token with no = sign (e.g. ARC-Seal: i=1; none) caused NULL to be passed to strip_whitespace(), triggering assert(string != NULL) → SIGABRT. That needed a separate guard.

3. The same issues also existed in the opendmarc-arcares.c parser, which was not covered here.

Closing in favour of #296, which addresses all of the above. Thanks again for the contribution!