registrar: add templatekeyword to mark a function as template keyword (API) _templateregistrarbase is defined as a super class of templatekeyword, for ease of adding template common features between "keyword", "filter" and "function". This patch also adds loadkeyword() to templatekw, because this combination helps to figure out how they cooperate with each other. Listing up loadkeyword() in dispatch.extraloaders causes implicit loading template keyword functions at loading (3rd party) extension. This change requires that "templatekeyword" attribute of (3rd party) extension is registrar.templatekeyword or so.

chgserver: do not keep repo object The current chgserver design is to use one server to handle multiple repos which has same [extensions] config. Previously the client uses --cwd / to avoid creating a repo object. Now we need to set repo to None before we have code to make "serve" command norepo when it's chg.

chgserver: invalidate the server if extensions fail to load Previously, if extensions fail to load, chg server will just keep working without those extensions. It will print a warning message but only if a new server starts. This patch invalidates the server if any extension failed to load, but still serve the client (hopefully just) once. It will help chg pass some test cases of test-bad-extension.t.

chgserver: add an explicit "reconnect" instruction to validate In some rare cases (next patch), we may want validate to do "unlink" without forcing the client reconnect. This patch addes a new "reconnect" instruction and makes "unlink" not to reconnect by default.