Warn about conflicting module options for $-variables

[tueda]

This PR makes FORM emit a warning when ModuleOption is used for a $-variable in a conflicting way within the same module:

Format 100;  * Default line width wraps warnings badly.
$a = 1;
ModuleOption local $a;
ModuleOption sum $a;
.end

test.frm Line 4 --> Warning: Conflicting module options for $-variable; later option ignored: $a

It also improves warnings for undefined $-variables:

Format 100;
ModuleOption local $a;
.end

test.frm Line 2 --> Warning: Undefined $-variable in module option; option ignored: $a

[coveralls]

coverage: 57.886% (-0.007%) from 57.893%
when pulling 788fb4f on tueda:pr/feat/warn-conflicting-dollar-moduleoptions
into eb24708 on form-dev:master.