Fix boolean logic in http_GetHdrToken

For quoted tokens, http_GetHdrToken should not try a case insensitive match
if the sensitive match fails
Move Tadd() to the only source file which uses it
Convert htc->pipeline from txt to two char* pointers
Convert htc->rxbuf from txt to dual char *
Make the various http_Get* functions return const char * to what they find.
Constify txt.

This is all Slink's fault.
now that txt is const, fm can be, too
Fix casts
Explicit cast back to pointer
Take a more competent stab a the casts which breaks builds in jenkins.
Move handling of PRIV_ variables to separate source file before it
gets too complicated.
Now that things are properly prepared: Implement PRIV_SESS and PRIV_REQ

Note that each VMOD only gets one of each, so that the same 'vmod_priv'
will appear on all VMOD functions which use PRIV_REQ and similarly
only a single vmod_priv appear for all PRIV_SESS uses in that VMOD.

Feedback welcome.
Add vct_tchar() (ref: rfc7230#section-3.2.6)
Update copyright statement.
Fix out-of-tree builds

Fixes #1479
revert ALLSPHINXOPTS from the previous commit