util: new string helpers introduced

This patch brings add_to_string() and construct_string() helpers.
They allow to create a string with variable amount of parameters in sprintf()
manner, but supporting string allocation (and reallocation if necessary)

v2:
1) Helpers were renamed to xstrcat() and xsprintf() respectively.
2) Added printf attributes to force compiler check

Signed-off-by: Stanislav Kinsburskiy <skinsbursky@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>