strbuf: provide CRLF-aware helper to read until a specified delimiter

strbuf: provide CRLF-aware helper to read until a specified delimiter

Many of our commands support reading input that is separated either via
newlines or via NUL characters. Furthermore, in order to be a better
cross platform citizen, these commands typically know to strip the CRLF
sequence so that we also support reading newline-separated inputs on
e.g. the Windows platform. This results in the following kind of awkward
pattern:

```
struct strbuf input = STRBUF_INIT;

while (1) {
	int ret;

	if (nul_terminated)
		ret = strbuf_getline_nul(&input, stdin);
	else
		ret = strbuf_getline(&input, stdin);
	if (ret)
		break;

	...
}
```

Introduce a new CRLF-aware helper function that can read up to a user
specified delimiter. If the delimiter is `\n` the function knows to also
strip CRLF, otherwise it will only strip the specified delimiter. This
results in the following, much more readable code pattern:

```
struct strbuf input = STRBUF_INIT;

while (strbuf_getdelim_strip_crlf(&input, stdin, delim) != EOF) {
	...
}
```

The new function will be used in a subsequent commit.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>