rc: make Read not ignore interrupts/errors (again)

Rc normally allows to interrupt unfinished commands,
like a command with an open { or ' character
(sometimes created without intention due
to unbalanced delimiters). If an interrupt signal is issued
on one of the next lines after writing e.g.
	% echo '

this should exit from the quoting block, and print a
line like
	rc: /dev/stdin: token EOF: syntax error.

Starting with rev 2474 this doesn't work anymore, as the
new readnb function, which takes care of nonblocked fds,
accidentally suppresses a return value of -1, which was
possible before and resulted in the EOF token.

The patch restructures the loop, so that normal
errors, i.e. errors different from EAGAIN, won't go
unnoticed. Perhaps the `goto' is not ok in terms of
style, but it seems that here it makes it clearer that
in the normal case it won't loop again, only in case
of errno==EGAIN.

[russcox_gmail.com, 2009-08-23 17:24:27]

thanks for fixing this.

codebot: apply
rc: make read not ignore interrupts/errors (again)

[old.codebot, 2009-08-23 17:25:44]

# codebot: apply

applied.