getcwd(2): saner logics around prepend_path() call

The only negative value that might get returned by prepend_path() is
-ENAMETOOLONG, and that happens only on overflow.  The same goes for
prepend_unreachable().  Overflow is detectable by observing negative
buflen, so we can simplify the control flow around the prepend_path()
call.  Expand prepend_unreachable(), while we are at it - that's the
only caller.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>