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>
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>