When using the force black background command-line option or configuration
option an invalid reset sequence is sent following a coloured output item 
in the CLI, the result is that the colour is not 'turned off' and continues
until the next non-default coloured text output.

A reset sequence is already defined in term.c, but the ast_term_reset
function doesn't use it, instead building it's own invalid sequence and 
returning that.

This patch changes that behaviour, removing the building of a reset sequence
and instead using the pre-built constant 'enddata' which is a suitable reset
sequence for this purpose.

ASTERISK-24896 #close
Reported by: Dan Tucny

Change-Id: I56323899123ae3264900389cae1f5b252aa3bf43