(url-history-hash-table): Initialize in declaration.

(url-history-hash-table): Initialize in declaration.
(url-history-parse-history): Don't reset the history.
(url-history-save-history): Create parent dir if necessary.
(url-history-save-history): Don't write the initialization of
url-history-hash-table into the history file.
(url-have-visited-url): Simplify since url-history-hash-table is non-nil.
(url-completion-function): Simplify.