API: libcrmcommon: Several crm_time_t args in iso8601.c are now const

This affects the following four public API functions:

• crm_time_as_string()
• crm_time_log_alias()
• crm_time_get_seconds()
• crm_time_get_seconds_since_epoch()

It was necessary to update all four of these functions in a single
commit.

1. crm_time_as_string(), crm_time_get_seconds(), and crm_time_get_seconds_since_epoch() all call the static function crm_get_utc_time().
2. crm_get_utc_time() calls crm_time_log_alias().
3. crm_time_log_alias() calls crm_time_as_string().
4. crm_time_as_string() can call crm_time_get_seconds() or crm_time_get_seconds_since_epoch().

So if any of them take a const crm_time_t pointer, all of them must be
updated to take a const pointer.

This is backward-compatible since a non-const pointer can be passed as a
const pointer argument.

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>