top: enable foreign users under 'u'/'U' task filtering

top: enable foreign users under 'u'/'U' task filtering

This commit will allow user filtering as long as there
is a valid number representing a potential user ID. It
will serve, for example, chroot environments where the
specific user may be unknown to a host, or vice versa.

(now that we know a '.' + 2 spaces is squeezed to one)
(everything's perfectly justified, but it's just luck)

Reference(s):
http://www.freelists.org/post/procps/top-enhancements-i-hope,1

Signed-off-by: Jim Warner <james.warner@comcast.net>