server: Convert com_lsatt() to lopsub.

This is the first afs subcommand which needs to pass a pattern list
to its callback. The new send_lls_callback_request() provides this
functionality. It serializes the parse result into a buffer and passes
this buffer to the callback.

Since there are non-lopsub commands which also pass a pattern list,
action_if_pattern_matches() is patched to receive the pattern
list either from the serialized parse result or in the old way via
pmd->data. To achieve this, a parse result pointer is added to struct
pattern_match_data. If this pointer is not NULL, we are dealing with
a subcommand that has been converted.

Since the ls subcommand has not been converted yet, lopsub will
regard "ls" as a uniqe abbreviation of the lsatt command, which
breaks t0004. To work around this, we deactivate prefix matching by
only accepting exact matches in run_command(). This workaround can
be removed after com_ls() has been converted.