Added SDP_ADMIN_PASSWORD_FILE variable in p4_vars.template, and also added a
default value in backup_functions.sh. Also added comments in p4_vars
explaining the 'set +u' bit.
Adding SDP_ADMIN_PASSWORD_FILE is an enabling change for an upcoming change
to mkrep.sh.
Updated p4login and p4login-super.sh to reference this variable.
Normalized p4login-super.sh to accept SDP instance parameter, which (as
with other scripts) is optional of SDP_INSTANCE is already defined, else
required. Also chmod +x p4login-super.sh.