Add battery_remaining function on OpenBSD

Implementation of a battery_remaining function which returns the
remaining battery time in HH:MM format. Linux function still needs
implementation.
Move common code to load_apm_power_info

slstatus.h

@@ -3,6 +3,7 @@
 /* battery */
 const char *battery_perc(const char *);
 const char *battery_state(const char *);
+const char *battery_remaining(const char*);
 
 /* cpu */
 const char *cpu_freq(void);