Cleanup and simplify get_reason() and related functions

Rename randreason() to fread_random() to better reflect its function,
and change it to a one-pass algorithm.  Have it write into a caller-provided
buffer instead of a static buffer.  Fixes a file leak when an empty
file is given to this function.

Change freadln() so that it never returns an empty line - it only returns
0 at end-of-file now.  This simplifies its callers.

Factor out some common code from the several get_reason()-type functions,
which greatly simplifies them.

source/dcc.c

@@ -4012,10 +4012,10 @@ FILE *f;
 	{
 		char *hst = NULL;
 		buffer = alloca(sb.st_size+1);
-		freadln(f, buffer);
+		freadln(f, buffer, sb.st_size + 1);
 		while(!feof(f))
 		{
-			if ((freadln(f, buffer)))
+			if ((freadln(f, buffer, sb.st_size + 1)))
 			{
 				if (*buffer == '#')
 					continue;