tools/xentop: reject invalid --delay and --iterations arguments

xentop parses -d/--delay and -i/--iterations with atoi(), so invalid
input is silently accepted with surprising results: a fractional delay
such as "-d 2.5" is truncated to 2; "-d abc" parses as 0, which in
batch mode turns the output loop into a busy loop; "-d -1" wraps to an
effective delay of about 136 years; and "-i 0" (or any unparsable
iterations count) decrements an unsigned counter from zero, running
for about 2^32 iterations.  atoi() also has undefined behaviour on
out-of-range input.

Parse both options with strtoull() instead, and reject anything that
is not a plain decimal integer in range: a sign, a fractional part,
trailing junk or overflow now produce an error and exit rather than a
silently wrong value.

Compatibility considerations: "--delay 0" remains accepted, since
updating as fast as possible is a plausible deliberate choice and
works today; "--iterations 0" is rejected, since running the loop
2^32 times cannot be what the caller meant.  The interactive 'D'
prompt already validates its input and is unchanged.  The only
previously useful invocation this breaks is a fractional delay, which
now fails loudly instead of silently rounding down - which is the
point of the change.

A patch documenting the --delay truncation was posted in 2010 but
never applied:
Link: https://lore.kernel.org/xen-devel/01ea26d2420e3562eb30.1292604768@chilopoda.uk.xensource.com/

Signed-off-by: Matthias Goergens <matthias.goergens@gmail.com>
Reviewed-by: Anthony PERARD <anthony.perard@vates.tech>

docs/man/xentop.1.pod

@@ -27,7 +27,7 @@ output version information and exit
 
 =item B<-d>, B<--delay>=I<SECONDS>
 
-seconds between updates (default 3)
+seconds between updates (default 3); must be a non-negative integer
 
 =item B<-n>, B<--networks>
 
@@ -55,7 +55,8 @@ output data in batch mode (to stdout)
 
 =item B<-i>, B<--iterations>=I<ITERATIONS>
 
-maximum number of iterations xentop should produce before ending
+maximum number of iterations xentop should produce before ending; must
+be a positive integer
 
 =item B<-z>, B<--dom0-first>