setopt: avoid integer overflows when setting millsecond values

... that are multiplied by 1000 when stored.

For 32 bit long systems, the max value accepted (2147483 seconds) is >
596 hours which is unlikely to ever be set by a legitimate application -
and previously it didn't work either, it just caused undefined behavior.

Also updated the man pages for these timeout options to mention the
return code.

Closes #1938