Make auto-completion case-sensitive and use a popup

Case-insensitive auto-completion makes it impossible to correct a
mistyped gpgconf -x to gpgconf -X. Switching to case-sensitive
auto-completion fixes this.

(cherry picked from commit 37db4bc9a7247e0711f441aef4c3bdbd73da20d3)

• GnuPG-bug-id: T7615