function as there is no gracefull way of handling failures.
Rename bb_getport to bb_lookup_port, allow a default port to be
specified so it always returns a correct value.
Modify ftpgetput/rdate/wget to use the new interface.
wget/rdate now use etc/services with a falback default value.
archive_xread can be replaced with bb_full_read, and archive_copy_file
with bb_copyfd*
bb_copyfd is split into two functions bb_copyfd_size and bb_copyfd_eof,
they share a common backend.