strbuf: give strbuf_getline() to the "most text friendly" variant

strbuf: give strbuf_getline() to the "most text friendly" variant

Now there is no direct caller to strbuf_getline(), we can demote it
to file-scope static that is private to strbuf.c and rename it to
strbuf_getdelim().  Rename strbuf_getline_crlf(), which is designed
to be the most "text friendly" variant, and allow it to take over
this simplest name, strbuf_getline(), so we can add more uses of it
without having to type _crlf over and over again in the coming
steps.

Signed-off-by: Junio C Hamano <gitster@pobox.com>