refactor: replace gliderlabs/ssh with golang.org/x/crypto/ssh (#38837)

Migrate away from this thin ssh wrapper module while adding more test
coverage.

Removes `sessionPartial`, which hand-copied the layout of a private
`gliderlabs/ssh` struct and reinterpreted a pointer to it via
`reflect.UnsafePointer` to reach the permissions of the authenticated
connection. The layout is unchecked, so an upstream field reorder would
mismatch silently.

The builtin server only needs the session channel with `exec` and
`shell`. Serving those on `x/crypto` drops the hack and the dependency,
since `PublicKeyCallback` returns permissions per key and `x/crypto`
assigns them only after verifying the signature.

Two benign behavior changes:

1. Internal session handler errors report exit status 1 rather than 0,
so a client no longer reads a failure as success.
1. An unusable host key is fatal at startup instead of being replaced by
an ephemeral one that would trigger an error at the client.

modules/ssh/init.go

@@ -11,11 +11,17 @@ import (
 	"strconv"
 	"strings"
 
+	"gitea.dev/modules/graceful"
 	"gitea.dev/modules/log"
 	"gitea.dev/modules/setting"
 	"gitea.dev/modules/util"
 )
 
+// builtinUnused informs our cleanup routine that we will not be using a ssh port
+func builtinUnused() {
+	graceful.GetManager().InformCleanup()
+}
+
 func Init() error {
 	if setting.SSH.Disabled {
 		builtinUnused()