This reverts commit c4c74c3883aa3122c0c877ca8dd7b26beb5cc4aa.

LibUV already gives an error for this, so this isn't needed.

Problem:    This doesn't give the right result: eval(string(v:true)). (Nikolay
            Pavlov)
Solution:   Make the string "v:true" instead of "true".
https://github.com/vim/vim/commit/f48aa160fdd7b8caa7678e1a2139244dd2bdc547

Problem:    The "trailing characters" error can be hard to understand.
Solution:   Add the trailing characters to the message.
https://github.com/vim/vim/commit/2d06bfde29bd3a62fc85823d2aa719ef943bd319

exepath and stdpath should respect shellslash and return path with
proper file separator.
Closes #13787

Problem:
Since right-click can now show a popup menu, we can provide messaging to
guide users who expect 'mouse' to be disabled by default. So 'mouse' can
now be enabled by default.

Solution:
Do it.
Closes #15521

These tests commonly fail in Debian with

    [  ERROR   ] test/functional/core/job_spec.lua @ 81: jobs append environment with pty #env
    test/functional/helpers.lua:198:
    ==============================================================================
    test/helpers.lua:73: Expected objects to be the same.
    Passed in:
    (table: 0x2aa4bcc1040) {
     *[1] = {
        [1] = 'notification'
        [2] = 'stdout'
       *[3] = {
          [1] = 0
         *[2] = {
            [1] = 'hello world abc' } } } }
    Expected:
    (table: 0x2aa4bcc0d70) {
     *[1] = {
        [1] = 'notification'
        [2] = 'stdout'
       *[3] = {
          [1] = 0
         *[2] = {
            [1] = 'hello world abc'
           *[2] = '' } } } }

Since the test is validating the "hello world abc" message, not the
newlines, accept either set of message sequences.

Problem:
- Since c57f6b28d71d #8519, sockets are created in ~/.local/… but XDG
  spec says: "XDG_RUNTIME_DIR: Must be on the local filesystem", which
  implies that XDG_STATE_DIR is potentially non-local.
- Not easy to inspect Nvim-created temp files (for debugging etc).

Solution:
- Store sockets in stdpath('run') ($XDG_RUNTIME_DIR).
- Establish "/tmp/nvim.user/" as the tempdir root shared by all Nvims.
- Make ok() actually useful.
- Introduce assert_nolog().

closes #3517
closes #17093