kernel: Skip sendfile tests that force async threads

The tests have to be skipped when the emulator running the tests does
not have any async threads.
Add hyperlinks to modules in reference manual for kernel application

This makes the pointers to other reference manuals consistent
throughout this page.
Store expanded config in application_controller:change_application_data

If the Config given to application_controller:change_application_data
included other config files, it was only expanded for already existing
(loaded) applications. If an upgrade added a new application which had
config data in an included config file, the new application did not
get correct config data.

This is now changed so config data will be expanded for all
applications.
Update kernel, stdlib and sasl appup files for Erlang/OTP-18
Merge branch 'siri/ac-change_app_data/OTP-11864' into maint

* siri/ac-change_app_data/OTP-11864:
  Store expanded config in application_controller:change_application_data
Update kernel, stdlib and sasl appup tests

Add test of update instructions from current major to current
minor. Earlier only previous major to current was tested.
Remove the pg module and related documentation

This module has been marked experimental for more than 15 years,
and has largely been superseded by the pg2 module from the kernel
application. The original pg also has no tests and has not been
updated in the last 15 years other than small maintenance edits
(like adding specs or replacing pid/1 by is_pid/1). It is pretty
unlikely that anyone uses it today and its presence is simply
confusing as people should be using pg2 anyway.
kernel: Fix erts_debug:size/1 to handle Map sizes
Merge branch 'lukas/kernel/fix_sendfile_no_async_tests/OTP-11903' into maint

* lukas/kernel/fix_sendfile_no_async_tests/OTP-11903:
  kernel: Skip sendfile tests that force async threads
Merge branch 'essen/remove-pg'

* essen/remove-pg:
  Remove the pg module and related documentation

OTP-11907
Merge branch 'siri/appups-18.0/OTP-11881'

* siri/appups-18.0/OTP-11881:
  Update kernel, stdlib and sasl appup files for Erlang/OTP-18
Merge branch 'siri/cuddle-with-tests' into maint

* siri/cuddle-with-tests:
  Fix regexp in release_handler test so versions are correctly replaced
  Update kernel, stdlib and sasl appup tests
  Minor update to test_server for finding old releases
Merge branch 'maint'
inet: fix typo in inet:getstat/2 doc

The word 'received' in 'send_dvi' option description seems to be
copy-pasted and not removed
Merge branch 'egil/fix-erts_debug-size/OTP-11923' into maint

* egil/fix-erts_debug-size/OTP-11923:
  erts: Update preloaded erts_internal.beam
  erts: Add spec for erts_internal:map_to_tuple_keys/1
  erts: Add testcase for erts_debug:size/1 Map terms
  kernel: Fix erts_debug:size/1 to handle Map sizes
  erts: Add erts_internal:map_to_tuple_keys/1
Merge branch 'maint'
Merge branch 'dz/inet_getstat_doc_typo_fix' into maint

* dz/inet_getstat_doc_typo_fix:
  inet: fix typo in inet:getstat/2 doc
Merge branch 'maint'
Teach code:del_path/1 to ignore superfluous slashes

The following sequence will NOT leave the code path unchanged:

  code:add_path("/some/app/"),
  .
  .
  .
  code:del_path("/some/app/")

The reason is that code:add_path/1 will normalize the path name
(removing the trailing slash), while code:del_path/1 does not
normalize the path before searching for it in the code path.
application_SUITE: Clean up the code after the test is done