Build mount_nfs(8) with WARNS=6.

Reviewed by:	rmacklem@
MFC after:	1 month
Sponsored by:	The FreeBSD Foundation
filedesc: fix missed comments about fdsetugidsafety

While here just note that both fdsetugidsafety and fdcheckstd take sleepable
locks.
Only put one CTASSERT() inside each macro to avoid compile issues.
The problem is that the __LINE__ macro is constant inside a macro and
results in identical assert statements when the compiler does not
support the static builtin assert function.

MFC:		3 days
Sponsored by:	Mellanox Technologies