bug#9101: timeout should use setitimer if available

[Paul Eggert]

setitimer has nanosecond resolution, which is better than the
one-second resolution that 'alarm' has.  timeout should use
setitimer if available, to take advantage of this.  On 64-bit
hosts, this has the additional advantage of increasing the
upper bound for timeouts from 2**31 seconds to 2**63 seconds
(about 68 years to about 292 billion years, which should be
long enough for most practical purposes :-).