tap: new option to change the string designating TAP diagnostic

tap: new option to change the string designating TAP diagnostic

Here we introduce a new option `--diagnostic-string' in our TAP
test driver, that allows the user to specify which string should
denote the beginning of a TAP diagnostic line.  This change is
not gratuitous, nor result if over-engineering: it is motivated
by real issues that have emerged during the use of TAP in the
Automake's own testsuite (see the commit `v1.11-1082-g9b967c2'
"testsuite: yet more use of TAP, and related extensions").

* doc/automake.texi (Use TAP with Automake test harness): Document
the new option.
(Incompatibilities with other TAP parsers and drivers): Report it
as a potential source of incompatibility.
* lib/tap-driver ($diag_string): New global variable, defaulting
to "#", and whose value can be changed ...
(Getopt::Long::GetOptions): ... by the newly recognized option
`--diagnostic-string'.
(handle_tap_comment): Subroutine removed, some of its simple logic
inlined ...
(main): ... in here, where now ...
(extract_tap_comment): ... this new subroutine is used.
($USAGE): Adjust.
* tests/tap-diagnostic.test: Make one check slightly stricter.
* tests/tap-diag-custom.test: New test.
* tests/Makefile.am (tap_other_tests): Add it.