grsecurity: Fix GRKERNSEC_PROC restrictions

Previously we were setting GRKERNSEC_PROC_USER y, which was a little bit
too strict. It doesn't allow a special group (e.g. the grsecurity group
users) to access /proc information - this requires
GRKERNSEC_PROC_USERGROUP y, and the two are mutually exclusive.

This was also not in line with the default automatic grsecurity
configuration - it actually defaults to USERGROUP (although it has a
default GID of 1001 instead of ours), not USER.

This introduces a new option restrictProcWithGroup - enabled by default
- which turns on GRKERNSEC_PROC_USERGROUP instead. It also turns off
restrictProc by default and makes sure both cannot be enabled.

Signed-off-by: Austin Seipp <aseipp@pobox.com>