strbuf_split*(): rename "delim" parameter to "terminator"

strbuf_split*(): rename "delim" parameter to "terminator"

The word "delimiter" suggests that the argument separates the
substrings, whereas in fact (1) the delimiter characters are included
in the output, and (2) if the input string ends with the delimiter,
then the output does not include a final empty string.  So rename the
"delim" arguments of the strbuf_split() family of functions to
"terminator", which is more suggestive of how it is used.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Jeff King <peff@peff.net>