Support fn keyword for external names

Fixes bpftrace#3177

This change adds `fn` to the list of external names that may be used to
index structures, etc. I had originally considered allowing `fn` (and
`config`) arbitrarily as an identifier within the body of a probe
(esentially extending `ident` to include CONFIG and SUBPROG within the
parser, and limiting `attach_point_def` accordingly), but since there is
already a strong split between internal and external identifiers, I went
with the simpler proposed solution.

Author: amscanne

src/parser.yy

@@ -650,6 +650,7 @@ keyword:
         |       SIZEOF        { $$ = $1; }
         |       UNROLL        { $$ = $1; }
         |       WHILE         { $$ = $1; }
+        |       SUBPROG       { $$ = $1; }
         ;
 
 ident:

tests/semantic_analyser.cpp

@@ -3104,6 +3104,8 @@ TEST(semantic_analyser, struct_member_keywords)
     "raw",
     "uprobe",
     "kprobe",
+    "config",
+    "fn",
   };
   for (auto kw : keywords) {
     test("struct S{ int " + kw + ";}; k:f { ((struct S*)arg0)->" + kw + "}");