MFC r278803, r278905:

MFC	r278803, r278905:
ulimit(3): Fix broken check.

The existing implementation had a broken comparison that could
overflow and return confusing values.  Replace this with a check
that avoids the overflow before it happens.

Consistently return a maximum value also on the case of negative
arguments since negative is considered an overflow and means
infinity for our current setrlimit().

New revamped version is credited to Bruce Evans.

CID:		1199295

git-svn-id: svn://svn.freebsd.org/base/stable/9@279130 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f