Convert underscores to dashes in some %define variable names.

For now, just api.push-pull and lr.keep-unreachable-states.
Maintain old names for backward compatibility.
* NEWS (2.5): Document.
* data/c.m4 (b4_identification): Update comment.
* data/yacc.c: Update access.
* doc/bison.texinfo: Update.
* etc/bench.pl.in (bench_push_parser): Update use.
* src/files.c (tr): Move to...
* src/getargs.c, src/getargs.h (tr): ... here because I can't
think of a better place to expose it.  My logic is that, for all
uses of tr so far, command-line arguments can be involved, and
getargs.h is already included.
* src/main.c (main): Update access.
* src/muscle_tab.c (muscle_percent_define_insert): Convert old
variable names to new variable names before assigning value.
* src/reader.c (reader): Update setting default.
* tests/calc.at: Update uses.
* tests/conflicts.at (Unreachable States After Conflict
Resolution): Update use.
* tests/input.at (%define enum variables): Update use.
(%define backward compatibility): New test group.
* tests/push.at: Update uses.
* tests/reduce.at: Update uses.
* tests/torture.at: Update uses.
(cherry picked from commit 812775a039)

Conflicts:

	data/c.m4
	etc/bench.pl.in
	src/parse-gram.c
	src/parse-gram.h
	tests/conflicts.at

data/c.m4

@@ -54,7 +54,7 @@ m4_define([b4_comment], [b4_c_comment($@)])
 # -----------------
 # Depends on individual skeletons to define b4_pure_flag, b4_push_flag, or
 # b4_pull_flag if they use the values of the %define variables api.pure or
-# api.push_pull.
+# api.push-pull.
 m4_define([b4_identification],
 [[/* Identify Bison output.  */
 #define YYBISON 1