Add environment variable access with $NAME syntax

Lexes $NAME as TK_ENVVAR, compiles reads as __getenv("NAME") calls
and writes ($NAME = expr) as __setenv("NAME", expr) calls. Runtime
functions wrap getenv/setenv/unsetenv with automatic tostring coercion.

llex.h

@@ -41,7 +41,8 @@ enum RESERVED {
   TK_DBCOLON, TK_EOS,
   TK_FLT, TK_INT, TK_NAME, TK_STRING,
   TK_COMMAND,
-  TK_COMMAND_INTERP
+  TK_COMMAND_INTERP,
+  TK_ENVVAR
 };
 
 /* number of reserved words */