notmuch: virtual-mailboxes should accept a limit

notmuch: virtual-mailboxes should accept a limit

* Fix issue #849

nm_uri_from_query() checks if there is a custom limit and includes it
in the URI. This keeps the limit intact after nm_normalize_uri()
terminates. Otherwise, the limit is stored within nm_normalize_uri()'s
temporary NmCtxData struct and lost upon termination.

Additionally, nm_normalize_uri() properly initializes tmp_ctxdata, which
sets the default db_limit value. This ensures db_limit is either
NmDbLimit or the custom limit. My earlier attempt to resolve #849 did
not initialize db_limit and suffered from random db_limit values.

The url_parse_query() check was removed from nm_normalize_uri() as
new_ctxdata() contains the same block of code. Error message numbering
has been updated the reflect the change.

* Remove redundant variable

Refactor per suggestions in code review to remove redundant variable and
update method signature.

The removal of the redundant variable prompt the need to change
tmp_ctxdata from NmCtxData to *NmCtxData. Calls to its fields have been
updated to reflect its new status.

* wipe the context before use