lexer: Rename LEX_ERROR_INTERACTIVE to LEX_ERROR_TERMINAL, change default

lexer: Rename LEX_ERROR_INTERACTIVE to LEX_ERROR_TERMINAL, change default

The LEX_ERROR_INTERACTIVE error handling mode really only makes sense for
reading from a terminal, where we know we're getting one line at a time
and not partial lines.  Unfortunately LEX_ERROR_INTERACTIVE was the default
and it was getting used as such in the GUI for reading from syntax files.
This meant that if there was an error in the GUI then the current buffer
(which might be any arbitrary chunk of syntax) was being discarded, which
caused terrible confusion.

This commit changes the default to LEX_ERROR_CONTINUE, which works
in a reasonable fashion regardless of the input source.  It also renames
LEX_ERROR_INTERACTIVE to LEX_ERROR_TERMINAL to make it clear that that's
the only reasonable use.

Reported by Thambu David <thambu@cmcvellore.ac.in>, archived at:
http://lists.gnu.org/archive/html/pspp-users/2013-08/msg00019.html