contrib/dev-util: make the cluster script work on macOS

[emelialei88]

🎯 The "Why" (Intent)

contrib/dev-util/cluster assumed a Linux host and an image that only ran a
host-built binary. On macOS init, run, tmux and test all broke: no systemd, no
docker group, no /proc, and a Mach-O binary Linux containers cannot execute.
Getting it working surfaced breakage that was never host-specific.

🛠️ The "What" (Critical Changes)

• update builds Comdb2 inside a node; the bind-mounted source means one build
  serves the cluster, in build-docker/ so it does not fight a host cmake cache.
• Linux-only checks — systemctl, docker group, /proc, sudo rm — are guarded
  by uname; figlet falls back to a plain banner.
• Fixes unrelated to the host: the image would not build where the host uid
  collides with the stock ubuntu account (uid 1000) in noble and later;
  start brought up nothing; setup ran a copy of this script frozen into the
  image; the client config went where cdb2api never reads it; and the persist=0
  tmpfs mounted on a literal /opt/bb:exec.
• db becomes startdb; stopdb, rmdb and set are new; status names the
  databases. Matching is by command line — comdb2 renames its main thread, so
  matching the process name misses a live server.
• tmux targets its own window, so panes cannot land in an unrelated attached
  session; gdb refuses under CPU emulation, where ptrace cannot read
  registers, and gdb/vg stop a running database first.

[roborivers]

Cbuild submission: Success ✓.
Regression testing: Success ✓.

The first 10 failing tests are:
sc_resume_logicalsc_generated **quarantined**
cdb2jdbc
consumer_non_atomic_default_consumer_generated **quarantined**
sc_downgrade [timeout] **quarantined**
phys_rep_tiered [timeout]
phys_rep_tiered_nosource_generated [timeout]
phys_rep_tiered_firstfile_generated [timeout]

[roborivers]

Cbuild submission: Success ✓.
Regression testing: Success ✓.

The first 10 failing tests are:
cdb2jdbc
reco-ddlk-sql **quarantined**
timepart_retro
consumer_non_atomic_default_consumer_generated **quarantined**
sc_downgrade [timeout] **quarantined**
phys_rep_tiered [timeout]
phys_rep_tiered_firstfile_generated [timeout]
phys_rep_tiered_nosource_generated [timeout]

[roborivers]

Cbuild submission: Error ⚠.
Regression testing: Success ✓.

The first 10 failing tests are:
sc_truncate [db unavailable at finish]
cdb2jdbc
ssl_san
consumer_non_atomic_default_consumer_generated **quarantined**
ssl_set_cmd
ssl_prefer
ssl_dbname
sc_downgrade [timeout] **quarantined**
reco-ddlk-sql [timeout] **quarantined**

[roborivers]

Cbuild submission: Error ⚠.
Regression testing: Success ✓.

The first 10 failing tests are:
sc_resume_logicalsc_generated **quarantined**
queuedb_rollover **quarantined**
cdb2jdbc
consumer_non_atomic_default_consumer_generated **quarantined**
sc_downgrade [timeout] **quarantined**

[roborivers]

Cbuild submission: Error ⚠.
Regression testing: Success ✓.

The first 10 failing tests are:
sc_resume
cdb2jdbc
consumer_non_atomic_default_consumer_generated **quarantined**
sc_downgrade [timeout] **quarantined**

[roborivers]

Cbuild submission: Error ⚠.
Regression testing: Success ✓.

The first 10 failing tests are:
sc_truncate_multiddl_generated [db unavailable at finish] **quarantined**
queuedb_rollover_noroll1_generated **quarantined**
cdb2jdbc
consumer_non_atomic_default_consumer_generated **quarantined**
sc_downgrade [timeout] **quarantined**