core: split WeeChat home in 4 directories, use XDG directories by default (issue #1285)

The 4 directories (which can be the same):

- config: configuration files, certificates
- data: log/upgrade files, local plugins, scripts, xfer files
- cache: script repository, scripts downloaded (temporary location)
- runtime: FIFO pipe, relay UNIX sockets

src/plugins/fifo/fifo-command.c

@@ -102,7 +102,8 @@ fifo_command_init ()
            "\n"
            "FIFO pipe is used as remote control of WeeChat: you can send "
            "commands or text to the FIFO pipe from your shell.\n"
-           "By default the FIFO pipe is in ~/.weechat/weechat_fifo\n"
+           "By default the FIFO pipe is called weechat_fifo and saved in the "
+           "WeeChat runtime directory (see /debug dirs).\n"
            "\n"
            "The expected format is one of:\n"
            "  plugin.buffer *text or command here\n"
@@ -110,7 +111,7 @@ fifo_command_init ()
            "\n"
            "For example to change your freenode nick:\n"
            "  echo 'irc.server.freenode */nick newnick' "
-           ">~/.weechat/weechat_fifo\n"
+           ">/run/user/1000/weechat/weechat_fifo\n"
            "\n"
            "Please read the user's guide for more info and examples.\n"
            "\n"