Merge from origin/emacs-25
6ee327d Add handle_user_signal_hook
47580e0 Avoid writing to purespace
0588be7 Remove unused variable
89e7483 * configure.ac: Find libxml2 headers in Xcode SDK dir on Darwin.
3b95e9c Use posix_openpt instead of openpty on Darwin
86312ff Document support for ':documentation' in Lisp mode
c930e75b Document new features of TeX mode
7c83d84 Clarify docs of hscroll in RTL text
4c8f8db Fix rendering of HTML pages that use character composition
a8d37ca Avoid some compiler warnings in w32.c
ce106f3de Undo ill-advised change
be0bba4 Unbreak completion in python-mode buffers