Fix p_endcopy.

For p_endcopy to work correctly, it must be the name of the next element
in struct proc after the end of the copy region, not the name of the
last element in the copy region. Currently, the last element
(p_elf_flags) is not being copied. In addition, the p_xexit and
p_xsig fields should not be copied on fork, so move them out of the
copy region.

Reviewed by: kib
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D10677