setitimer(2): consolidate copyin(9), input validation, input conversion

For what are probably historical reasons, setitimer(2) does not
validate its input (itv) immediately after copyin(9).  Instead, it
waits until after (possibly) performing a getitimer(2) to copy out the
state of the timer.

Consolidating copyin(9), input validation, and input conversion into a
single block before the getitimer(2) operation makes setitimer(2)
itself easier to read.  It will also simplify merging the critical
sections of setitimer(2) and getitimer(2) in a subsequent patch.

This changes setitimer(2)'s behavior in the EINVAL case.  Currently,
if your input (itv) is invalid, we return EINVAL *after* modifying the
output (olditv).  With the patch we will now return EINVAL *before*
modifying the output.  However, any code dependent upon this behavior
is broken: the contents of olditv are undefined in all setitimer(2)
error cases.

ok millert@

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