Further sanitize types:
(unsigned) int -> bool / (u)intX_t where possible

Forgot to constify decode_time.h in last commit, do this now.
Comment fix for set_mode_live().
Directly fill fields of 'newtime' instead of going through tmp* variables first.
Check 'newtime.tm_mday' against 'newtime' instead of 'time'.

This was probably a leftover from before 'newtime' was added.