Adapt tag completion for changes in Emacs 25.

Stop storing module:tag entries in the completion table for definitions
in hrl-files.  These entries make no sense since hrl-files are not
modules.

Add test-case for tag completion in erlang-test.el.

Ensure prog-mode is defined for the sake of Emacs 23 and XEmacs.

Extracting data from debug info in BEAM files is parallelized.

The speed-up is not outstanding. If memory is not an issue, adding
`{spawn_opt, [{min_heap_size, <large number>}]}' to the option list of
xref:start/2 can make a (small) difference.

Reading of modules from a directory is parallelized. Attempts to
parallelize all modules when adding a release have been rejected since
the gain showed to be small compared to the increased complexity of
the code.

Using a process for coordinating the reading has not been attempted.

This reverts commit e020f75c10410a6943cd055bfa072a2641eab7da.

Add current module to non external functions calls.
Also improves finding record and macro definitions.

The right order is defined by find-tag-tag-order which is a list of
predicate functions.  Tags matched by the first function shall be
visited first.  After that tags matched by the seconds function shall be
visited and so on.

Previous behavior was to visit tags that match any of the functions in
the order they appear in the TAGS file.

Reported by Dan Gudmundsson.

* dgud/tools/emacs-xref/PR-1051:
  Add xref support in erlang.el

Conflicts:
	lib/tools/emacs/erlang.el

In GNU Emacs 25 xref will be introduced.  It is a framework for cross
referencing commands, in particular commands for finding definitions.
It does not replace etags.  It rather resides on top of it and
provides user-friendly commands.  The idea is that the user commands
should be the same regardless of what back-end does the actual finding
of definitions.

Add to the xref commands awareness of the module:tag syntax in a
similar way that is already done for the old etags commands.

Xref completion support is not included in this commit.

Remove all compilation warnings for GNU Emacs 24.5 and current 25
(4ffec91).

Remove XEmacs incompatibility in
erlang-font-lock-exported-function-name-face.

Add file erlang-test.el with a single unit test.

Add TAGS to .gitignore.

Inside type specs, line-initial commas shall align with open curly
braces. This also applies to record definitions with type specs for
the fields.

For example:

    -type foo() :: { a :: integer()
                   , b :: integer()
                   }.

For type specs for functions, the commas shall align with the opening
parenthese. So do a closing parenthese when put on a new line.

For example:

    -spec foo( X :: integer()
             , Y :: integer()
             ) -> integer().

This does not affect coding styles that don't put commas at the
beginning of lines.