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>
Return proper errno code when mapping UNIX socket address.

Path mapping now can fail if pathname component doesn't exist,
so this condition must be handled correctly when mapping UNIX socket address.

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