Backport htslib/kseq.h parsing performance improvement (PR #1227)

Apply the main part of PR samtools/htslib#1674. Pysam does use kseq itself,
so it is worth applying this now without waiting for the next HTSlib release
to be imported.

Co-Authored-By: Fabian Klötzl <fabian@kloetzl.info>

htslib/htslib/kseq.h

@@ -110,8 +110,8 @@
 				} else break; \
 			} \
 			if (delimiter == KS_SEP_LINE) {  \
-				for (i = ks->begin; i < ks->end; ++i)  \
-					if (ks->buf[i] == '\n') break; \
+				unsigned char *sep = memchr(ks->buf + ks->begin, '\n', ks->end - ks->begin); \
+				i = sep != NULL ? sep - ks->buf : ks->end; \
 			} else if (delimiter > KS_SEP_MAX) { \
 				for (i = ks->begin; i < ks->end; ++i) \
 					if (ks->buf[i] == delimiter) break; \