Add explicit "# key" directive to legacy snippet collection before deprecating filenames-as-triggers

snippets/cc-mode/do

@@ -1,4 +1,5 @@
 #name : do { ... } while (...)
+# key: do
 # --
 do
 {

snippets/cc-mode/for

@@ -1,4 +1,5 @@
 #name : for (...; ...; ...) { ... }
+# key: for
 # --
 for (${1:int i = 0}; ${2:i < N}; ${3:++i})
 {

snippets/cc-mode/if

@@ -1,4 +1,5 @@
 #name : if (...) { ... }
+# key: cc-modeif
 # --
 if (${1:condition})
 {

snippets/cc-mode/inc

@@ -1,3 +1,4 @@
 #name : #include "..."
+# key: inc
 # --
 #include "$1"

snippets/cc-mode/inc.1

@@ -1,3 +1,4 @@
 #name : #include <...>
+# key: inc
 # --
 #include <$1>

snippets/cc-mode/main

@@ -1,4 +1,5 @@
 #name: int main(argc, argv) { ... }
+# key: main
 # --
 int main(int argc, char *argv[])
 {

snippets/cc-mode/once

@@ -1,4 +1,5 @@
 #name : #ifndef XXX; #define XXX; #endif
+# key: once
 # --
 #ifndef ${1:_`(upcase (file-name-nondirectory (file-name-sans-extension (buffer-file-name))))`_H_}
 #define $1

snippets/cc-mode/struct

@@ -1,4 +1,5 @@
 #name : struct ... { ... }
+# key: struct
 # --
 struct ${1:name}
 {