Ensure that libnv can be used when kern.trap_enotcap=1.

libnv used fcntl(fd, F_GETFL) to test whether fd is a valid file
descriptor. Aside from being racy, this check requires CAP_FCNTL
rights on fd. Instead, use fcntl(fd, F_GETFD), which does not require
any capability rights.

Also remove some redundant fd_is_valid() checks to avoid extra system
calls; in many cases we were performing this check immediately before
dup()ing the descriptor.

Reviewed by: cem, oshogbo (previous version)
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D17963