Allow installable builds to be installed into a specific path (#10923)

* Pass path to install()

It was dirty that it would re-get $HOME there anyway.

* Import wcs2osstring

* Allow installable builds to use a relocatable tree

If you give a path to `--install`, it will install fish into a
relocatable tree there, so

PATH/share/fish contains the datafiles
PATH/bin/fish contains the fish executable
PATH/etc/fish is sysconf

I am absolutely not sold on that last one - the way I always used
sysconfdir is that it is always /etc. This would be easy to fix but
should probably also be fixed for "regular" relocatable builds (no
idea who uses them).

An attempt at #10916

* Move install path into "install/" subdir

* Disable --install harder if not installable

doc_src/cmds/fish.rst

@@ -40,9 +40,11 @@ The following options are available:
 **-i** or **--interactive**
     The shell is interactive.
 
-**--install**
+**--install[=PATH]**
     When built as self-installable (via cargo), this will unpack fish's datafiles and place them in ~/.local/share/fish/install/.
     Fish will also ask to do this automatically when run interactively.
+    If PATH is given, fish will install itself into a relocatable directory tree rooted at that path.
+    That means it will install the datafiles to PATH/share/fish and copy itself to PATH/bin/fish.
 
 **-l** or **--login**
     Act as if invoked as a login shell.