lib: split ul_default_shell() from shells.c into default_shell.c

lib: split ul_default_shell() from shells.c into default_shell.c

Move ul_default_shell() and UL_SHELL_* flags into a new
lib/default_shell.c and include/default_shell.h pair, and add the
new file to libcommon (autotools and meson).

ul_default_shell() has no dependency on libeconf -- it simply checks
$SHELL, passwd, and falls back to _PATH_BSHELL. Separating it from
shells.c (which depends on libeconf for /etc/shells enumeration)
allows binaries that only need the default shell lookup (flock,
script, scriptlive, more, exec_shell.c users) to avoid unnecessary
libeconf linkage.

Update callers that only use ul_default_shell() to include
default_shell.h instead of shells.h.

Signed-off-by: Karel Zak <kzak@redhat.com>