Enhance the log command by providing clearer feedback to users. Now, if no log file is specified, the message explicitly suggests using log <file> for better clarity. Additionally, a message is printed when logging is enabled, improving user awareness.

Before: Ambiguous log command feedback

$ ./qtest
cmd> log
No log file given
cmd> log temp.txt
cmd> new
l = []
cmd> quit
Freeing queue
$

After: Clear and informative log command messages

$ ./qtest
cmd> log
No log file given. Use 'log <file>'. 
cmd> log temp.txt
Logging enabled: temp.txt 
cmd> new
l = []
cmd> quit
Freeing queue
$
$ cat temp.txt
l = []
Freeing queue
$

Change-Id: I355f3f13bd959ddecc2893d202642accc565fb8a