files: add ability to set callbacks for files (v7)

Here is nothing interecting. If a file can't be dumped by criu,
plugins are called. If one of plugins knows how to dump the file,
the file entry is marked as need_callback. On restore if we see
this mark, we execute plugins for restoring the file.

v2: Callbacks are called for all files, which are not supported by CRIU.
v3: Call plugins for a file instead of file descriptor. A few file
descriptors can be associated with one file.
v4: A file descriptor is opened in a callback. It's required for
    restoring anon vmas.
v5: Add a separate type for unsupported files
v6: define FD_TYPES__UNSUPP
v7: s/unsupp/ext (external)

Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
zdtm: test dumping character devices with help of custom callbacks

Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
cr_service(): don't use pr_perror()

Since sd_listen_fds() doesn't set errno when returning a value > 1,
it doesn't make sense to use pr_perror(). Use pr_err() instead.

While at it, remove the period from the log message.

[v2: fix function names]

Signed-off-by: Kir Kolyshkin <kir@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
tab-at-eol cleanup

Remove whitespace at EOL (found by git grep '	$')
(the character before $ is real tab, typed in shell using Ctrl+V Tab)

To people using vim, I'd suggest adding the following code to ~/.vimrc:

let c_space_errors = 1
highlight FormatError ctermbg=darkred guibg=darkred
match FormatError /\s\+$\|\ \+\t\|\%80v.\|\ \{8\}/

Signed-off-by: Kir Kolyshkin <kir@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
criu-log.h: fit macros in 80 columns

Before this patch, backslash was at 81th column which makes the text
twice longer on a standard 80 col terminal, which is quite annoying.

Signed-off-by: Kir Kolyshkin <kir@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
seize: add more detailed comment

Why we do several attempts to freeze tasks.

Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
ptrace: Add comment about early interrupt

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
ptrace: the task state is restored automatically

It's a feature of PTRACE_SEIZE.  So we need to do something, only
if we want to change the state.

[xemul: If task _was_ in stopped state before dump and we want them
 to stay alive after dump, the existing code queues one more STOP
 to it. This affects subsequent dump, as we seize a stopped task
 with STOP in queue.

 One more item in TODO list -- support stopped tasks with STOP in
 queue :)
]

Signed-off-by: Andrew Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>