Try to avoid parse_{time,units,flags} symbol conflicts

On Debian stretch systems PAM modules linked against Heimdal run into
symbol conflicts with parse_time() in systemd libraries.  We redefine
parse_time() and friends to rk_parse_time(), ... while keeping the old
names for ABI compatibility with existing clients.

The legacy names should eventually be droppped.

lib/roken/parse_time.h

@@ -46,6 +46,11 @@
 #endif
 #endif
 
+#define parse_time rk_parse_time
+#define unparse_time rk_unparse_time
+#define unparse_time_approx rk_unparse_time_approx
+#define print_time_table rk_print_time_table
+
 ROKEN_LIB_FUNCTION int ROKEN_LIB_CALL
 parse_time (const char *s, const char *def_unit);