Handle large numbers when parsing/printing a duration

The isccfg_duration_fromtext() function is truncating large numbers
to 32 bits instead of capping or rejecting them, i.e. 64424509445,
which is 0xf00000005, gets parsed as 32-bit value 5 (0x00000005).

Fail parsing a duration if any of its components is bigger than
32 bits. Using those kind of big numbers has no practical use case
for a duration.

The isccfg_duration_toseconds() function can overflow the 32 bit
seconds variable when calculating the duration from its component
parts.

To avoid that, use 64-bit calculation and return UINT32_MAX if the
calculated value is bigger than UINT32_MAX. Again, a number this big
has no practical use case anyway.

The buffer for the generated duration string is limited to 64 bytes,
which, in theory, is smaller than the longest possible generated
duration string.

Use 80 bytes instead, calculated by the '7 x (10 + 1) + 3' formula,
where '7' is the count of the duration's parts (year, month, etc.), '10'
is their maximum length when printed as a decimal number, '1' is their
indicator character (Y, M, etc.), and 3 is two more indicators (P and T)
and the terminating NUL character.

lib/isccfg/include/isccfg/duration.h

@@ -24,7 +24,7 @@
 
 ISC_LANG_BEGINDECLS
 
-#define DURATION_MAXLEN 64
+#define CFG_DURATION_MAXLEN 80
 
 /*%
  * A configuration object to store ISO 8601 durations.
@@ -76,6 +76,10 @@ isccfg_duration_toseconds(const isccfg_duration_t *duration);
  * - Months will be treated as 31 days.
  * - Years will be treated as 365 days.
  *
+ * Notes:
+ *\li	If the duration in seconds is greater than UINT32_MAX, the return value
+ * 	will be UINT32_MAX.
+ *
  * Returns:
  *\li	The duration in seconds.
  */