Vaild values are 'auto' or 'manual', default 'auto'. With 'auto' you
get the auto-generated SYNOPSIS in the VMOD docs. With 'manual', the
SYNOPSIS is just skipped, so the VMOD author has to write it.

This makes logging the local and peer address in BackendOpen
independent of the socket type.

Add arg names to synopsis

73ba50b8 was incomplete and I confused
vsc_seg and vsc staring at this trivial code

Ref: #2576

PRIV_* arguments are unnamed, so vmodtool names their argstruct members
arg%d

Also add a few extra asserts, in an effort to chase down #2591

If we fail a stream early from VCL via return (fail) after sending a
request body, the h2 sess will be stuck waiting forever.

Spotted by:	dridi