Move fish_greeting to a function

[faho]

This adds a "fish_greeting" function that prints the variable of the
same name.

In doing so, it makes $fish_greeting default to a global
variable (this is of little cost because of the _ builtin)

This means that:

• We have fewer universal variables by default
• If we change the default greeting people will actually get
  it (unless they have a leftover universal, of course)
• If the user changes their language the variable changes with it

TODOs:

• Changes to fish usage are reflected in user documentation/manpages.
• [N/A] Tests have been added for regressions fixed
• User-visible changes noted in CHANGELOG.rst

[zanchey]

The "it's a function! and a variable!" nature always gives me the heebie-jeebies, but this sounds sensible to me.

[exploide]

When launching a fish for the first time, I usually only need to do two things: First, add the sbin directories to the PATH and second, disable the fish_greeting message. What is the preferred way for doing the latter, now that it is a global variable instead of a universal? Prior to that I just ran set fish_greeting.

[faho]

What is the preferred way for doing the latter, now that it is a global variable instead of a universal? Prior to that I just ran set fish_greeting.

It defaults to a global variable. You can still set it as a universal:

set -U fish_greeting

Or set it as a global in config.fish (this is executed after), or customize the function.

[ridiculousfish]

This LGTM! Don't forget to update CHANGELOG.