nanosleep(2): Plug a memory disclourse and be POSIX-compliant

nanosleep(2): Plug a memory disclourse and be POSIX-compliant

The nanosleep1() called by sys_nanosleep() would only update rmt on
EINTR, but it can also return other errors, such as EINVAL.  And in that
case, sys_nanosleep() would update the user-space rmtp by copying
garbage from its stack frame.  This is not only a kernel memory
disclourse, but also not POSIX-compliant.

Fix sys_nanosleep() to update rmtp only on EINTR.  In addition, zero the
kernel rmt variable for double safety.

Obtained-from: FreeBSD (r315510)
Reviewd-by: dillon