fileio: add new helper call read_line() as bounded getline() replacement

read_line() is much like getline(), and returns a line read from a
FILE*, of arbitrary sizes. In contrast to gets() it will grow the buffer
dynamically, and in contrast to getline() it will place a user-specified
boundary on the line.

src/basic/fileio.h

@@ -101,3 +101,5 @@ int link_tmpfile(int fd, const char *path, const char *target);
 int read_nul_string(FILE *f, char **ret);
 
 int mkdtemp_malloc(const char *template, char **ret);
+
+int read_line(FILE *f, size_t limit, char **ret);