Improve build support

support debos ARGS being pre-set:

ARGS='-c 4' ./build.sh # ...

expose hostname config as H option:

./build.sh -H mob-3 # ...

allow enabling contrib component of Debian/Mobian repository as C option:

./build.sh -S testing -C # ...

split Debian versus Mobian suite specification, via x option (following https://blog.mobian-project.org/posts/2021/03/15/unstable-distro/#so-how-is-mobian-unstable-different-from-debian-unstable):

./build.sh -x bullseye -S unstable # ...