Parser tweaks

[defnull]

Multiple parser improvements

fix: Allow CRLF in front of first boundary even in strict mode

Browsers do not do this, but some HTTP client libaries do and it's
technically allowed.

fix: Fail on invalid first boundary instead of skipping the first segment

As per spec, the start boundary must be at position zero, or start with CRLF
to separate it from the preamble. Boundaries are forbidden in segment
bodies, but not in the preamble. This means that a preamble can actually
contain the boundary, as long as it does not start with CRLF. This is
nonsense, so let's ignore the spec here. A preamble that contains the
boundary is so rare and suspicious that we assume a broken or malicious
client and fail fast, even in non-strict mode. This is way better than
silently skipping the first segment and loosing data.

fix: Accept tiny (1 byte) chunks and arbitrary chunk borders

There were edge cases where a very uncommon chunk border would break the
parser. This is fixed and properly tested now.

fix: Fail if message did not contain the boundary at all.

In strict mode, we fail very fast (because the message is either empty, or
the message does not start with a boundary. In non-strict mode, we fail
at the end when the parser closes in PREAMBLE state.

[defnull]

I'll update the changelog, then this can be merged I think.