In file_open_image(), mount non-native files *before* looking up a
file-proc. Previously, we'd only mount the file after the initial
lookup, and fail to perform a second lookup if the mount succeeded,
leaving us with a NULL file-proc and a subsequent segfault.
Additionally, simplify the rest of the remote-file code-path.
(cherry picked from commit 0829dba97f)
c38a23b90c