cleanup: Fix fread() and fwrite() error handling

Cleaned up some of the incorrect uses of fread() and fwrite(), mostly in
the format modules. Neither of these functions will ever return a value
less than 0, which we were checking for in some cases.

I've introduced a fair amount of duplication in the format modules, but
I plan to change how format modules work internally in a subsequent
patch set, so this is simply a stop-gap.

Change-Id: I8ca1cd47c20b2c0b72088bd13b9046f6977aa872

main/manager.c

@@ -6458,6 +6458,12 @@ static int get_input(struct mansession *s, char *output)
 	}
 
 	ao2_lock(s->session);
+	/*
+	 * It is worth noting here that you can all but ignore fread()'s documentation
+	 * for the purposes of this call. The FILE * we are working with here was created
+	 * as a result of a call to fopencookie() (or equivalent) in tcptls.c, and as such
+	 * the behavior of fread() is not as documented. Frankly, I think this is gross.
+	 */
 	res = fread(src + s->session->inlen, 1, maxlen - s->session->inlen, s->session->f);
 	if (res < 1) {
 		res = -1;	/* error return */