Do not hardcode paths to the default editor/pager

Use "vi" instead of "/usr/bin/vi" and "less" instead of "/usr/bin/less".
Hardcoding absolute paths is a bad idea:

    $ uname -rsv
    Linux 3.0-ARCH #1 SMP PREEMPT Tue Aug 30 07:32:23 UTC 2011
    $ which less
    /bin/less

The "$PATH" environment variable will almost always have a better idea
of where these binaries are located.

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

src/calcurse.h

@@ -100,6 +100,9 @@
 #define DPID_PATH        DIR_NAME DPID_PATH_NAME
 #define NOTES_DIR        DIR_NAME NOTES_DIR_NAME
 
+#define DEFAULT_EDITOR   "vi"
+#define DEFAULT_PAGER    "less"
+
 #define ATTR_FALSE	0
 #define ATTR_TRUE	1
 #define ATTR_LOWEST	2