Catch compressor execution errors.

Catch compressor execution errors.

* src/misc.c (write_fatal_details): Move to buffer.c
* src/buffer.c (write_fatal_details): Close the archive and wait for
the compressor program to terminate in order to catch eventual execution
errors.
* src/system.c (sys_child_open_for_compress): Ignore SIGPIPE so tar will
not silently terminate when unable to write to the compressor.
* tests/comperr.at: New file.
* tests/Makefile.am: Add comperr.at
* tests/testsuite.at: Include comperr.at