When csh/tcsh aborts the initial host-info marker probe before emitting
PI_HOST_PROBE=, the previous fallback returned os/shell unknown without
ever checking whether sh -lc still worked. That kept ssh:// rejecting a
POSIX-capable host (P2 from PR #3722 review).
The marker-missing fallback now runs probeTransferShell before returning.
If sh/bash/zsh round-trips the transfer marker, the fallback carries
transferShell and derives os from that probe's uname -s output. Windows
compat unames (MINGW/MSYS/Cygwin/Windows) still classify as windows so
ssh:// keeps refusing Windows hosts.
Added osFromUname coverage for Linux, GNU/Linux, Darwin, Windows compat
unames, and unknown payloads.
roboomp
f16d618ca2