These are versions of H2_Send_Get/H2_Send_Rel that can be called while
already holding the lock.

Maintain stream 0 window limits via h2->req0.

****heads in the gcc&llvm projects who cannot cooperate on
usability features ?

Fixes #2449

This commit enforces that a HEADERS frame without an END_HEADERS flag
must be followed by a CONTINUATION frame on the same stream.

Sending interleaved/out of sequence header or continuation frames will
now be treated as a PROTOCOL_ERROR, as per rfc7540.

This also fixes the panics seen in #2387.

Fixes: #2387

Remove duplication to some extent by moving common return keywords
to a separate paragraph.

Add fail and vcl(label) keywords

Fix some typos

Since we decoupled the client and backend side, restarts are completely
unrelated to backend requests.

Related to #2405

req->restarts only gets incremented in cnt_restart, so it will never
exceed max_restarts here.

Related to #2405

max_restarts is the number of allowed restarts

Related to #2405

On platforms where umem is available (specifically Solaris descendents), we want
to make umem the default allocator for performance reasons and also for vtc coverage
while still keeping the malloc type as fallback option.

This reverts commit 766dee06.

Conflicts:
	bin/varnishd/mgt/mgt_main.c

varnishd sources now use cache_priv.h as primary #include, and that
includes cache.h, rather than the other way around.