Fix broken xmllint validation.

An older version of xmllint failed to validate a broken
URL.

The URL led to nowhere, so this also fixes a few dangling
links.

Use olink & targetptr instead of link & linkend, as is the
convention in the rest of the manual.

Give internal links a bit longer names (as is also convention
in the rest of the manual).

The final version of odnoklassniki patch did not include the
most important part - delayed free of salloc memory.

Make a quick & dirty fix to the point.

Set signal handler for all signals, not just SIGTERM.
Print signal name to the log on exit.

Add constants for signal count. Rename. Coding style.

Do not use atexit() handlers, except the global tarantool_free()
function, which does nothing in the snapshot process.

Use check_symbol_exists(), not check_function_exists(),
to discover fdatasync(), since apparently on some BSD systems
the symbol is present in the library but not in the headers
(I hope not vice versa).

Don't use check_prototype_definition, since it's not available
in CMake 2.6.