move more validation for special targets to linter warnings

[mcandre]

• Use of special targets as prerequisites (except .WAIT)
• Use of prerequisites in inappropriate special target rules
• Use of commands in inappropriate special target rules

This provides clearer error messages.

This somewhat simplifies the grammar.

However, any special targets which reject commands, cannot be linted so easily for use of the blank/reset semicolon (;) syntax. We could extend the AST with a boolean to indicate whether an inline command was used.