Extend electric-pair-mode actions to < and >, and also to ( and ) in literals

Extend electric-pair-mode actions to < and >, and also to ( and ) in literals

* lisp/progmodes/cc-cmds.el (c-electric-lt-gt): Actuate electric-pair-mode if
a < or > is typed in a context where this is meaningful (#include, or
template).
(c-electric-paren): Allow electric-pair-mode activity in a comment or string.

* lisp/progmodes/cc-defs.el (c-make-keywords-re): Fix a bug where lists of
source symbols could get overwritten when parameter adorn is set to
'appendable.

* list/progmodes/cc-langs.el (c-cpp-include-key): New lang const and var.