Pageant: move extension list out into header file.

That's a part of the protocol spec (ish), so it should be somewhere
reasonably sensible rather than buried in the middle of a source file.

pageant.c

@@ -609,15 +609,6 @@ static bool reencrypt_key(PageantKey *pk)
     return true;
 }
 
-#define PUTTYEXT(base) base "@putty.projects.tartarus.org"
-
-#define KNOWN_EXTENSIONS(X)                             \
-    X(EXT_QUERY, "query")                               \
-    X(EXT_ADD_PPK, PUTTYEXT("add-ppk"))                 \
-    X(EXT_REENCRYPT, PUTTYEXT("reencrypt"))             \
-    X(EXT_REENCRYPT_ALL, PUTTYEXT("reencrypt-all"))     \
-    /* end of list */
-
 #define DECL_EXT_ENUM(id, name) id,
 enum Extension { KNOWN_EXTENSIONS(DECL_EXT_ENUM) EXT_UNKNOWN };
 #define DEF_EXT_NAMES(id, name) PTRLEN_DECL_LITERAL(name),

pageant.h

@@ -242,3 +242,15 @@ int pageant_reencrypt_key(struct pageant_pubkey *key, char **retstr);
 int pageant_reencrypt_all_keys(char **retstr);
 int pageant_sign(struct pageant_pubkey *key, ptrlen message, strbuf *out,
                  uint32_t flags, char **retstr);
+
+/*
+ * Definitions for agent protocol extensions.
+ */
+#define PUTTYEXT(base) base "@putty.projects.tartarus.org"
+
+#define KNOWN_EXTENSIONS(X)                             \
+    X(EXT_QUERY, "query")                               \
+    X(EXT_ADD_PPK, PUTTYEXT("add-ppk"))                 \
+    X(EXT_REENCRYPT, PUTTYEXT("reencrypt"))             \
+    X(EXT_REENCRYPT_ALL, PUTTYEXT("reencrypt-all"))     \
+    /* end of list */