Remove "squeezing" code in pager

The "squeezing" code attempted to force completions to fit
onto a single screen, but could render them unreadable.
Let's just get rid of it.
Remove pre-1.22 migration support in fish_config_interactive

This removes some gnarly and probably broken code that attempts
to upgrade from fish 1.21 to fish 1.22. Fixes #1714.
Make set_color take multiple colors, and choose the best supported one

As suggested in #1323
Clean up vared error message output

Fixes #1323
Make universal variables cache the path to the variable file
Put the universal variables notifier named pipe in XDG_RUNTIME_DIR

This is where named pipes belong.
document behaviour of arrays that are split on import
__fish_systemctl_*: use systemctl to generate the list of units

Closes #1804.

Note that if systemd is not running, the completion will fail.
Make a string into a reference, to save a copy
Use a directory in /tmp instead of ~/.config/fish for named pipe path

The home directory may not be able to support named pipes, e.g. if it's
NFS. This puts the file in a secured directory in /tmp
add license for tmux code added in d944add
Replace some std::string with wcstring to reduce copying
Move struct stat checks in configure

Tests for POSIX_C_SOURCE value need to be done before any actual checks
Vim bindings doc: describe modes better and explain some non-vim bindings
update keybindings docs
Makefile: install locale files to $localedir
__fish_print_packages: add support for zypper

Closes #1787.
abbr/web_config: support space-delimited abbreviations

Support for space-delimited abbreviations was added to the expansion
parser in fbade198; this commit extends that support to the user-facing
tools, and documents the space-separated behaviour. Equals-delimited
abbreviations are expected to be removed before the next release.

Work on #731.
abbr.fish: improve support for corner cases

Handle unusual cases ('=abc', ' =abc') better - regression from
8e8e6314due to a7bab7b.

Work on #731.
abbr.fish: escape the output of abbr --show

Allows abbreviations containing embedded newlines, etc., to be displayed
and exported properly.

Work on #731.