Fix tftpboot which was broken by my botched WARNSfy in last year.
Also add comments that mention libsa tftp requires network device socket
in f_devdata in struct open_file, from spz@ in PR port-sparc

Fix tftpboot which was broken by my botched WARNSfy in last year.
Also add comments that mention libsa tftp requires network device socket
in f_devdata in struct open_file, from spz@ in PR port-sparc64/46652.
Briefly tested tftpboot and nfsboot on Ultra5.

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WARNSfy and fix dumb bugs noticed by warnings.

Put the external functions from net.c in it's own header file and
include it, and do some other minimal adjustments to bring this
back to a buildable state.

OK'ed by martin@