nvidia-detect: use booleans for has_intel and has_nvidia

Signed-off-by: Philip J Perry <phil@elrepo.org>
nvidia-detect: check_xorg_abi_compat() can return abi_compat as a boolean

abi_compat returned from the function check_xorg_abi_compat can be a boolean

Signed-off-by: Philip J Perry <phil@elrepo.org>
nvidia-detect: remove unnecessary braces

Signed-off-by: Philip J Perry <phil@elrepo.org>
nvidia-detect: use braces for nested if-else statements

Signed-off-by: Philip J Perry <phil@elrepo.org>