Header container moves from a vector of raw lines to a vector
of string pairs representing name/value pairs in headers.  For
incoming headers, we normalize the name to lowercase and trim
it.  Values are only left-trimmed.  Outgoing headers are left
as-is.  Simple find() method for the common case, forward and
reverse iterators for those few who need to do it themselves.
The HTTP status line (e.g. 'HTTP/1.1 200 Ok') is no longer treated
as a header to be returned to caller.  Unit tests, as usual,
were a bear but they absolutely ensured outgoing HTTP header
conformance after the change.  Grunt work paid off.

LLTextureFetch was also given a second options structure
for texture fetches.  Same as the original but with header return
to caller requested.  Baked textures should use this, the other
20,000 texture fetch requests should continue to use the original.