Add 'HttpRangeRequestsDisable' debug setting to inhibit use of 'Range:' header.

Intended for users with bad networking gear or twitchy ISPs, if set to
True, forces plain GET requests to asset servers for textures and meshes.
This change kicked off a slight refactor in the mesh repository code which
made it resilient against unexpected 200's and responses not covering
the requested start range.  There's still too much data copying in the
Mesh code (always has been).  Would love to fix that and get rid of the
monolithic temp buffer.  Cleaned up white space damage caused by unnamed
linden who likes to drag his magical editor through code.