Change dont_resolve_final_symlink argument of sbox_map_path() to flags.

This refactoring changes a single dont_resolve_final_symlink flag
into more generic flags argument, allowing more flags to be added later.

SBOX_MAP_PATH_DONT_RESOLVE_FINAL_SYMLINK value of flags argument
is used instead of non-zero dont_resolve_final_symlink argument.

Signed-off-by: Oleg Girko <oleg.girko@jolla.com>
Add SBOX_MAP_PATH_ALLOW_NONEXISTENT flag to sbox_map_path().

This flag allows path resolution to succeed even if non-final
components in the pathname refer to nonexistent directories.

This is needed for emulating bindtextdomain() function
which can accept pathname refering to nonexistent directory
without returning an error.

Signed-off-by: Oleg Girko <oleg.girko@jolla.com>