`get_host_by_addr_or_name()` should try with a (hardcoded?) environment variable suffix

[kivkiv12345]

This would make it easy to switch from one satellite context to another.
Let's imagine this environment variable is called $(SAT).

The setup:

# First satellite
var set SAT sat0

node 518
node add pdu0$(SAT)
run -v 0 $(__FILE_DIR__)/modules/pdup4.csh
node 519
node add pdu0-r$(SAT)
run -v 0 $(__FILE_DIR__)/modules/pdup4.csh

node 530
node add -p pdu0$(SAT) -c 5 hk$(SAT)
run -v 0 $(__FILE_DIR__)/modules/obcp4-hk.csh



# second satellite
var set SAT sat1

node 582
node add pdu0$(SAT)
run -v 0 $(__FILE_DIR__)/modules/pdup4.csh
node 583
node add pdu0-r$(SAT)
run -v 0 $(__FILE_DIR__)/modules/pdup4.csh

node 594
node add -p pdu0$(SAT) -c 5 hk$(SAT)
run -v 0 $(__FILE_DIR__)/modules/obcp4-hk.csh

The payoff:

# Set satellite context to `sat0`
var set SAT sat0
# Switching to `hk` on `sat0`
node hk
ping

# Set satellite context to `sat1`
var set SAT sat1
# Switching to `hk` on `sat1`
node hk
ping

I could imagine a small slash command that automatically sets this environment variable:

# Set satellite context to `sat0`
sat sat0
# Switching to `hk` on `sat0`
node hk
ping

# Set satellite context to `sat1`
sat sat1
# Switching to `hk` on `sat1`
node hk
ping

The current satellite context should probably also be shown in the prompt somehow.

Tab completion of nodes should also be updated to not show satellites to not show nodes outside the current subnet.

I don't personally have a strong opinion on the flexibility that satellite context switching gives.
But if we decide to support it, I want it done in the least cumbersome way.

This proposal is not mutually exclusive with specific init files for specific satellites, but I think it's fair to say it argues more for:

csh_inits/t1.csh
csh_inits/t2.csh

Rather than:

csh_inits/t1_sat0.csh
csh_inits/t1_sat1.csh

csh_inits/t2_sat0.csh
csh_inits/t2_sat1.csh

[kivkiv12345]

This should probably also apply to node add itself.