value-pairs: replace trash stack with own stack implementation

Because of the g_trash_stack and its usage problems could arise when
the strict-aliasing compile option was set leading to bad value-pairs
expansion. With the own implementation the bogous usage of g_trash_stack
is supposed to be eliminated.

Signed-off-by: Daniel Gados <gdani@balabit.hu>
Merge pull request #106 from gdani/f/value_pairs_bugfix

value-pairs: replace trash stack with own stack implementation