Add elaborate_expr_param_bit_ and pass param refs for variable selects

For variable parameter selects (indexed and bit) pass the value being
selected as a parameter reference instead of just a numeric constant
so it is easy to get the underlying parameter information.

Move the parameter bit select code into its own routine and cleanup
the remaining general code.

This should not effect the VVP code, but the ivl_expr_parameter(<expr>)
routine can be used to get at the actual parameter information in the
vlog95 target.
Fix space issue.
vlog95: Better unsupported select warnings and variable parameter bit select

Since parameters are now passed by reference use that information to print
the parameter name in a select vs trying to figure it out by searching the
scope looking for a parameter with the same file, line and value information.

Only print the $signed() may need to be removed messages for a select
expression that will actually be cast to signed.

Use the actual parameter information to warn that -pallowsigned=1 is needed
for a parameter when the LSB > MSB.
Merge branch 'master' of git://github.com/steveicarus/iverilog