remove world read/write permissions from /dev/diag

/dev/diag has world read/write permissions which is considered
a security risk. Remove these permissions.

Add qcom_diag group for relevant processes. Access to diag requires
processes to be part of a new "qcom_diag" group. Adding qmuxd to be
part of this new group to be able to log messages to diag.

Change-Id: Id7b12958320dd1ed303ebb527621e16f42086e08
readme: update kernel branch