[SV 64185] Clarify handling of directive lines starting with TAB

[SV 64185] Clarify handling of directive lines starting with TAB

It's clear that this change causes too many problems to be made
without warning.  Revert the change disallowing conditional lines to
start with TAB.

Instead, generate warnings whenever a directive line begins with a
TAB character.  Make this change for all directives, not just
conditional directives: define, undefine, export, unexport, vpath,
load, include, etc.

* NEWS: Update the backward-compatibility warning.
* src/read.c (eval): Track whether the line starts with a TAB.
If so then whenever we recognize a directive, emit a warning.
Revert the previous change for this bug.
(parse_var_assignment): Accept a file location if the line begins
with TAB; show a warning if we discover a directive.
(conditional_line): Warn about lines starting with TAB.
* tests/scripts/...: Add tests to verify warnings for initial TAB.