Implement the "wait" checkpoint action.

This action instructs tar to pause until given signal is delivered.
The newer genfile tool uses this action for communication between
genfile and tar in exec mode. This eliminates race conitions and
speeds up the tests based on exec mode.

* doc/tar.texi: Document changes.
* paxutils: Upgrade.
* src/checkpoint.c: Implement the wait action.
* src/common.h (decode_signal): New proto.
* src/tar.c (decode_signal): New function.
(set_stat_signal): Rewrite.
* tests/dirrem01.at: Adjust genfile and tar command line arguments
to use the new feature.
* tests/dirrem02.at: Likewise.
* tests/filerem01.at: Likewise.
* tests/filerem02.at: Likewise.
* tests/grow.at: Likewise.
* tests/sptrcreat.at: Likewise.
* tests/sptrdiff00.at: Likewise.
* tests/sptrdiff01.at: Likewise.
* tests/truncate.at: Likewise.

tests/sptrdiff01.at

@@ -35,10 +35,7 @@ echo creating
 tar --sparse -vcf bar foo baz
 echo comparing
 genfile --run --checkpoint 5 --length 221278210 --truncate foo -- \
- tar --checkpoint=1 \
-     --checkpoint-action=echo='Write checkpoint %u' \
-     --checkpoint-action=sleep=1 \
-     --sparse -vdf bar 
+ tar --sparse -vdf bar 
 ],
 [1],
 [creating