Honor "TMPDIR" environment variable

Replace all hardcoded paths referring to "/tmp" with a new function that
honors the "TMPDIR" environment variable as well as P_tmpdir and uses
"/tmp" as a fallback.

Thanks-to: Erik Saule <esaule@bmi.osu.edu>
Signed-off-by: Lukas Fleischer <calcurse@cryptocrack.de>

src/calcurse.h

@@ -854,6 +854,7 @@ long         now (void);
 char        *nowstr (void);
 long         mystrtol (const char *);
 void         print_bool_option_incolor (WINDOW *, unsigned, int, int);
+const char  *get_tempdir (void);
 char        *new_tempfile (const char *, int);
 void         erase_note (char **, enum eraseflg);
 int          parse_date (char *, enum datefmt, int *, int *, int *,