Limit unbindkeyset to the keyset or chain that was named

[burnhamrobertp]

Fixes #3141. Chains are stored under their last keyset, so the list for a key holds every chain ending with it, and unbindkeyset erased that whole list. unbindkeyset a therefore also dropped a,a and a,a,a.

It now parses a keychain rather than a single keyset and removes only the entries whose chain matches exactly, which also means you can name a chain to get at just that one. unbindkeyset a,a previously removed nothing at all, since the comma failed to parse.

Comparing with == keeps it from reaching across modifier combinations, same as #3136 does for unbind, so naming any+j won't take out a bound ctrl+j.

A key that only appears mid-chain still isn't reachable this way, which I think is right for a command that names a keyset - sc_a,sc_l,sc_a isn't the keyset sc_l. If we ever want "clear anything involving this key" that feels like its own command.

Nothing in BAR looks affected: it only unbindkeysets f11 variants and one dynamic keybind, and no chains end in f11.

Verified headless - naming the plain key, the 2-chain and the 3-chain each drop only their own binds, mid-chain is left alone, modifier matching is exact both ways, and all actions sharing a named keyset still go together.

Used Claude to help investigate and write this.