common: Strip trailing slashes from the homedir.

* common/homedir.c (default_homedir): Strip trailing slashes.
(gnupg_set_homedir): Ditto.

is_gnupg_default_homedir() does not ignore trailing slashes when
comparing directory names. This can lead to multiple agents started
on the same directory if the homedir was specified with --homedir or
GNUPGHOME without or with a number of slashes.

We now make sure that the home directory name never ends in a
slash (except for the roo of course).

• GnuPG-bug-id: T3295
• Signed-off-by: Werner Koch <wk@gnupg.org>