add bsd fgetln function

optimized to avoid allocation and return lines directly out of the
stream buffer whenever possible.

include/stdio.h

@@ -172,6 +172,10 @@ int getw(FILE *);
 int putw(int, FILE *);
 #endif
 
+#ifdef _BSD_SOURCE
+char *fgetln(FILE *, size_t *);
+#endif
+
 #ifdef _GNU_SOURCE
 int asprintf(char **, const char *, ...);
 int vasprintf(char **, const char *, va_list);

src/internal/stdio_impl.h

@@ -57,7 +57,7 @@ struct __FILE_s {
 	int waiters;
 	void *cookie;
 	off_t off;
-	void *dummy4;
+	char *getln_buf;
 	void *mustbezero_2;
 	unsigned char *shend;
 	off_t shlim, shcnt;

src/stdio/fclose.c

@@ -16,6 +16,7 @@ int fclose(FILE *f)
 	r = fflush(f);
 	r |= f->close(f);
 
+	if (f->getln_buf) free(f->getln_buf);
 	if (!perm) free(f);
 	
 	return r;

src/stdio/fgetln.c

+#include "stdio_impl.h"
+
+char *fgetln(FILE *f, size_t *plen)
+{
+	char *ret = 0, *z;
+	ssize_t l;
+	FLOCK(f);
+	ungetc(getc_unlocked(f), f);
+	if ((z=memchr(f->rpos, '\n', f->rend - f->rpos))) {
+		ret = (char *)f->rpos;
+		*plen = ++z - ret;
+		f->rpos = (void *)z;
+	} else if ((l = getline(&f->getln_buf, (size_t[]){0}, f)) > 0) {
+		*plen = l;
+		ret = f->getln_buf;
+	}
+	FUNLOCK(f);
+	return ret;
+}