prctl.2: Document (uncomment) PR_SET_MM options in Linux 3.5

prctl.2: Document (uncomment) PR_SET_MM options in Linux 3.5

Some of the PR_SET_MM options were merged to vanilla kernel
later, and appeared in Linux 3.5. Those are:

   - PR_SET_MM_ARG_START
   - PR_SET_MM_ARG_END
   - PR_SET_MM_ENV_START
   - PR_SET_MM_ENV_END
   - PR_SET_MM_AUXV
   - PR_SET_MM_EXE_FILE

This patch merely uncomments description and error codes
for these options, which were present but commented out,
and removes the appropriate FIXME marks.

Signed-off-by: Kir Kolyshkin <kir@openvz.org>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>