[ng] vars: don't warn about non-POSIX names or GNU make function calls

[ng] vars: don't warn about non-POSIX names or GNU make function calls

This change has the collateral effect of making the warning
category 'portability-recursive' obsolete.

* doc/automake.texi: Adjust and update.
* lib/Automake/ChannelDefs.pm: Don't register the warning channel
'portability-recursive' anymore.
(switch_warning): Don't handle 'portability-recursive' category
anymore.  Related simplifications.
* lib/Automake/Variable.pm ($_VARIABLE_CHARACTERS): Remove.
($_VARIABLE_PATTERN): Likewise.
($_VARIABLE_RECURSIVE_PATTERN): Likewise.
(check_variable_expansions): Likewise.
(define): Don't call it.
* automake.in (read_am_file): Likewise.
(handle_options): Don't try to set/unset warnings in the category
'portability-recursive'.
* tests/test-driver-custom-xfail-tests.test: Remove now redundant
use of '-Wno-portability-recursive' in AUTOMAKE_OPTIONS.
* tests/canon.test: Adjust grepping of automake stderr.
* tests/canon5.test: Likewise.
* tests/silent6.test: Remove checks dealing with enabling/disabling
of 'portability-recursive' warnings.
* tests/gmake-vars.test: New test.
* tests/vars3.test: Remove as obsolete.
* tests/silent-nowarn.test: Likewise.
* tests/dollarvar.test: Likewise.
* tests/dollarvar2.test: Likewise.
* tests/extra-portability3.test: Likewise.
* tests/doc-parsing-buglets-colneq-subst.test: Remove as redundant.
* tests/list-of-tests.mk: Update.