pgsql: On some platforms, pg_usleep isn't interruptible by signals; fix

Log Message:
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On some platforms, pg_usleep isn't interruptible by signals; fix
archiver to behave per original coder's expectation on these machines.
We already know this everywhere else AFAICT.

Modified Files:
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pgsql/src/backend/postmaster:
pgarch.c (r1.24 -> r1.25)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/pgarch.c.diff?r1=1.24&r2=1.25)