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.

include/misc.h

@@ -130,8 +130,7 @@ ChannelList *	BX_prepare_command (int *, char *, int);
 	void		set_nslookupfd(fd_set *);
 	long		print_nslookup(fd_set *);
 	void		auto_nslookup();
-	int		freadln(FILE *, char *);
-
+int freadln(FILE *, char *, size_t);
 
 	void		BX_close_socketread(int);
 	int		BX_add_socketread(int, int, unsigned long, char *, void (*func_read)(int), void (*func_write)(int));