Add new warnings invalid-var and invalid-ref

Add new warnings invalid-var and invalid-ref

The "invalid-var" warning triggers if the makefile attempts to assign
a value to an invalid variable name (a name containing whitespace).
The "invalid-ref" warning triggers if the makefile attempts to
reference an invalid variable name.  Both new warnings have a default
action of "warn".

* NEWS: Add these new warnings.
* doc/make.1: Document them in the man page.
* doc/make.texi (Warnings): Document them in the user's manual.
* src/warning.h: Add enum values for the new warning types.
* src/main.c (initialize_warnings): Initialize the new warnings.
* src/variable.h (undefine_variable_in_set, undefine_variable_global):
Ask callers to provide a struct floc specifying where the variable
is undefined.
* src/read.c (do_undefine): Pass floc when undefining.
* src/variable.c (check_valid_name): If invalid-var is enabled, check
the variable name.
(define_variable_in_set): Call it.
(undefine_variable_in_set): Ditto.
(check_variable_reference): If invalid-ref is enabled, check the
variable reference.
(lookup_variable): Call it.
(lookup_variable_in_set): Ditto.
* tests/scripts/options/warn: Add tests for the new warning types.