Use strncasecmp() for case-insensitive comparison

Instead of converting everything to upper case and then using strncmp(),
use strncasecmp() which does case-insensitive comparison out of the box.

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

src/calcurse.h

@@ -977,7 +977,6 @@ char *new_tempfile(const char *, int);
 int parse_date(const char *, enum datefmt, int *, int *, int *, struct date *);
 int parse_time(const char *, unsigned *, unsigned *);
 int parse_duration(const char *, unsigned *);
-void str_toupper(char *);
 void file_close(FILE *, const char *);
 void psleep(unsigned);
 int fork_exec(int *, int *, const char *, const char *const *);