Expose "missingtexture" and "missingalpha" to all the texture functions.

These were supported in the underlying OIIO texture options, but there
was no way to set them from OSL.

The idea is that by passing a "missingtexture" (and optionally
"missingalpha") parameter to texture(), texture3d(), or environment(),
it instructs the texture system to substute that color for textures that
are missing or broken, and NOT treat them as an error.  If this optional
argument is not present, a missing or broken texture will be treated as
an error, as usual.