Fix ftp hang in libfetch after rS313974

[bapt]

Summary

In rS313974, a netrcfd field was added to struct url, to prepare for
capsicumization. The field should be initialized to -2, but this is
only done in fetchMakeURL(), not in fetchParseURL(). Since the ftp
part of libfetch only calls the latter, the netrcfd field is left at
its default value of zero, and fetching ftp:// URLs then hangs after the
server displays its banner, before authentication starts.

Fix this by initializing the netrcfd to -2 in fetchParseURL().

[bapt]

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