Support all accumulating SSH config options instead of re-implementing override behavior

[EhabY]

Currently we manually handle SSH config merging, but this requires re-implementing SSH's "first match wins" override behavior ourselves.

Proposed approach

Instead of manual merging, let the SSH client handle it natively:

• Place override configs first in the file (SSH uses "first match wins")
• Keys without values (for non-accumulating options) can be used to remove the entry if we still want to maintain that behavior
• For accumulating options, pass entries as-is from both configs without combining them

This keeps files leaner while avoiding custom override logic.

Accumulating SSH options to support

These options accumulate across multiple declarations rather than following "first match wins":

Option	Behavior
IdentityFile	Tried in sequence
CertificateFile	Tried in sequence
SetEnv	All values sent to server
SendEnv	Accumulates (prefix with - to clear)
LocalForward	Multiple tunnels
RemoteForward	Multiple tunnels
DynamicForward	Multiple SOCKS proxies

Additionally, these use space-separated values: GlobalKnownHostsFile, UserKnownHostsFile, PermitRemoteOpen, CanonicalDomains.

Reference

OpenBSD ssh_config(5)