(fillcache): Rewrite. Old version failed to check success of lseek(),

(fillcache): Rewrite.  Old version failed to check success of lseek(),
couldn't cope with interrupted or short reads (mostly harmless, as
disk reads commonly complete), and left an empty cache after a failed
read.
(ef_open): Use fillcache() instead of read().
(ef_read): Fail if fillcache() fails to read at least one element.
Old version copied garbage to its caller and claimed success.