* lisp/progmodes/sql.el: Use cl-lib and lexical-binding; various cleanup.

* lisp/progmodes/sql.el: Use cl-lib and lexical-binding; various cleanup.
(sql-signum): Remove.  Use `cl-signum' instead.
(sql-read-passwd): Remove; use read-passwd instread.
(sql-get-login-ext): Use read-string.
(sql-get-login): Use dolist and pcase.
(sql--completion-table): Rename from sql-try-completion.
Use complete-with-action.
(sql-mode): Don't change abbrev-all-caps globally.
(sql-connect): Don't rely on dynamic scoping for `new-name'.
(sql-postgres-completion-object): Initialize vars in their `let'.
(sql-comint-sybase, sql-comint-sqlite, sql-comint-mysql)
(sql-comint-solid, sql-comint-ms, sql-comint-postgres)
(sql-comint-interbase): Use a single append, without setq.
(sql-comint-linter): Same, and unwind-protect the LINTER_MBX var.