parser: prohibit redefinitions of symbols and verify existance on use

parser: prohibit redefinitions of symbols and verify existance on use

This patch fixes some incorrect behaviour of symbolic variables:

- prohibit redefining an existing symbol
- verify existance of a symbolic variable during parsing instead of evaluation

The second point makes sure we don't allow recursive definitions (var = $var),
which lead to a crash due to stack exhaustion.

Signed-off-by: Patrick McHardy <kaber@trash.net>