chmod: ugo must refer to unaltered perms.

Within the same clause, permissions given by ugo must refer to
the unaltered permissions, before the target bits were masked
out, otherwise self-assignment like o=o just clears the
permissions. The other self-referential perm is X: it checks
for existing x permissions. That works with the current
value.

* sysif.c (chmod_wrap): Keep the old permissions in a new loop
variable called oldm. The u, g and o perms refer to oldm
rather than to the updated value in cmode. When we hit a
comma, we update oldm to the current value. The code for
this is now in one place with a goto.

* tests/018/chmod.tl: New test case that fails in the
absence of this fix. Test cases confirming that X
refers to the current permissions.

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