debian: create fastrpc group and add user to it in postinstall

[Shoudi Li (shoudil)]

The postinstall script creates a 'fastrpc' system group via
systemd-sysusers, falling back to 'addgroup --system', and adds the
installing user to it.

$SUDO_USER is preferred over $USER because during package installation
$USER resolves to root; $SUDO_USER captures the real caller when
invoked via 'sudo apt install ./pkg.deb'.

[Shoudi Li (shoudil)]

Checked on device:

qcom@qcom:$ sudo dpkg -i fastrpc-support_1.0.6-2_arm64.deb
Selecting previously unselected package fastrpc-support.
(Reading database ... 86267 files and directories currently installed.)
Preparing to unpack fastrpc-support_1.0.6-2~_arm64.deb ...
Unpacking fastrpc-support (1.0.6-2~) ...
Setting up fastrpc-support (1.0.6-2~) ...
Creating group 'fastrpc' with GID 987.
adsprpcd.service is a disabled or a static unit, not starting it.
adsprpcd_audiopd.service is a disabled or a static unit, not starting it.
cdsp1rpcd.service is a disabled or a static unit, not starting it.
cdsprpcd.service is a disabled or a static unit, not starting it.
gdsp0rpcd.service is a disabled or a static unit, not starting it.
gdsp1rpcd.service is a disabled or a static unit, not starting it.
sdsprpcd.service is a disabled or a static unit, not starting it.
qcom@qcom:$ groups qcom
qcom : qcom sudo audio video users dmaheap fastrpc
qcom@qcom:$ groups
qcom sudo audio video users fastrpc dmaheap