Use a dynamic method to print appointments to stdout

Add a flexible helper function print_apoint() and use it whenever we
print appointments to stdout. This reduces the number of copy-pasted
code and eventually allows for specifying custom format strings.

Following format specifiers are supported:

* s: Print the start time of the appointment as UNIX time stamp
* S: Print the start time of the appointment using the "hh:mm" format
* d: Print the duration of the appointment in seconds
* e: Print the end time of the appointment as UNIX time stamp
* E: Print the end time of the appointment using the "hh:mm" format
* m: Print the description of the item
* n: Print the name of the note file belonging to the item

Signed-off-by: Lukas Fleischer <calcurse@cryptocrack.de>

src/calcurse.h

@@ -922,6 +922,7 @@ int          fork_exec (int *, int *, const char *, char *const *);
 int          shell_exec (int *, int *, char *);
 int          child_wait (int *, int *, int);
 void         press_any_key (void);
+void         print_apoint (const char *, long, struct apoint *);
 
 /* vars.c */
 extern int               col, row;