Avoid rereading non-regular profile files

Avoid rereading non-regular profile files

If a profile file is a special device such as a pipe, then reloading
it will cause us to discard its contents.  Repurpose the first
PROFILE_FILE flag bit to indicate that the file should not be
reloaded.  In profile_update_file_data_locked(), set the flag on the
first load if stat() doesn't indicate a regular file, and check the
flag before testing whether we need to perform any subsequent reload.

Later on in profile_update_file_data_locked(), specifically unset the
DIRTY flag rather than unsetting all flags other than SHARED, so that
we don't clear the NO_RELOAD flag.

ticket: 8651