IceWM is automatically installed as a fallback DE. So we want to default
to one of the DEs the user actually chose.

Using the new session_files() subroutine allows us to eliminate
get_session_file() and get_session_file_with_extension().

'chksession -L' no longer returns any numbers, so prepending $wm_number
to the session name written to ~/.dmrc has no effect.

Some callers of any::sessions() expected it to return a list of session
names, other callers expected it to return a list of base file names,
leading to various bugs in the autologin handling. So provide two new
subroutines, session_files(), to provide a map of names to files, and
session_names() to return a sorted list of names. Remove sessions() to
avoid confusion in future. Fix up the users accordingly.

GDM now reads the session setting from /var/lib/AcoountsService/users/<name>,
so we need to create/update that file.

There are now multiple session files in /usr/share/xsessions and in
/usr/share/wayland-sessions for each DE, so the old hash table was
both broken and inadequate. Replace with a subroutine that uses
case-insensitive regex matching, which should be more future-proof.

Since Perl 5.28, Perl has used the thread-safe uselocale() functions
from the C standard library when setting the locale. Once a thread-safe
locale setting has been made, the Locale::gettext functions will use
those settings, not the global locale settings. So calling the drakx
c::setlocale() and c::init_setlocale() functions, which still use the
underlying setlocale() function from the C standard library normally
has no effect. However, prior to Perl 5.38, there was a bug in Perl
that could cause the main thread to revert to using the global locale
settings, and it it would seem this bug was being triggered by the
stage2 installer, meaning it still properly translated the UI after
the language selection step. Now we have a newer version of Perl, this
is no longer so, and the UI contiues to be in English after the user
has selected a different language (mga#34656).

The POSIX::setlocale() function affects the thread-safe locale, so can
be used to fix this bug.

This patch removes the c::setlocale() function to prevent reintroduction
of this bug, and moves the c::init_locale() function to the common module.

Both the old and new implementations of sessions() used chdir() to change
to the directory being searched and then tried to restore the current
working directory using the return value of chdir(). But chdir() returns
a boolean indicating success or failure, so that could never work.

Commit 98978485f changed sessions() to search two directories, but
didn't merge the results into a single list. Actually the code was
not valid Perl, so sessions() always returned an empty list.