(MacOSX 10.3) by adding a custom test for _POLL_EMUL_H_ which is
defined in poll.h on some MacOSX 10.3 systems.
not all 10.3 systems have poll.h, so only do the poll() test if at
least one of the header files is found.
Various changes, including:
* forbid mget of filenames that aren't in or below the local cwd.
* improve auto-fetch transfers
* improve www/proxy authentication support
* improve http response header parsing
* change UCB-licensed code from 4-clause to 3-clause
