There's a subtle difference between fork and vfork... when doing the
dup() before the exec, with cfork we are still dealing with the
parent process "possessed" by the forked process briefly inbetween
the vfork() and the exec().
That matters because when we close the duplicated fds for the stdxxx
pipes, with vfork we're closing the fd we still want to hold in the
parent process.
The vfork optimized spawn, stdxxx and terminal handling in the cgi
implementation is quite mature and sophisticated, and useful for
other things unrelated to cgi. Break it out into its own public
api under LWS_WITH_SPAWN, off by default.
Expand it so the parent wsi is optional, and the role and protocol
bindings for stdxxx pipes can be set. Allow optional sul timeout
and external lws_dll2 owner for extant children.
Remove inline style from minimal http-server-cgi
