fixed compiler warnings and program name in error message

Building ddate produced the following warnings:
```
ddate.c: In function 'main':
ddate.c:43: warning: incompatible implicit declaration of built-in function 'exit'
ddate.c: In function 'ending':
ddate.c:93: warning: incompatible implicit declaration of built-in function 'malloc'
```
Additionally, executing ddate with an improper argument (e.g. `ddate badthing`) produced the following error message (with no trailing newline):
```
Syntax: DiscDate [month day year]
```

I corrected the warnings by including stdlib.h, changed "DiscDate" to "ddate", and added a newline to the output.