Fix quoted argument in emacsclient-mail.desktop Exec key

Fix quoted argument in emacsclient-mail.desktop Exec key

Apparently the emacsclient-mail.desktop file doesn't conform to the
Desktop Entry Specification at
https://specifications.freedesktop.org/desktop-entry-spec/desktop-entry-spec-latest.html#exec-variables
which says about the Exec key:

| Field codes must not be used inside a quoted argument, the result of
| field code expansion inside a quoted argument is undefined.

However, the %u field code is used inside a quoted argument of the
Exec key in both the [Desktop Entry] and [Desktop Action new-window]
sections.
* etc/emacsclient-mail.desktop (Exec): The Desktop Entry
Specification does not allow field codes like %u inside a quoted
argument. Work around it by passing %u as first parameter ($1)
to the shell wrapper.
* etc/emacsclient.desktop (Exec): Use `sh` rather than `placeholder`
as the command name of the shell wrapper.  (Bug#60204)