coredump: avoid deadlock when passing processed backtrace data

coredump: avoid deadlock when passing processed backtrace data

We would deadlock when passing the data back from the forked-off process that
was doing backtrace generation back to the coredump parent. This is because we
fork the child and wait for it to exit. The child tries to write too much data
to the output pipe, and and after the first 64k blocks on the parent because
the pipe is full. The bug surfaced in Fedora because of a combination of four
factors:
- 87707784c70dc9894ec613df0a6e75e732a362a3 was backported to v251.5, which
  allowed coredump processing to be successful.
- 1a0281a3ebf4f8c16d40aa9e63103f16cd23bb2a was NOT backported, so the output
  was very verbose.
- Fedora has the ELF package metadata available, so a lot of output can be
  generated. Most other distros just don't have the information.
- gnome-calendar crashes and has a bazillion modules and 69596 bytes of output
  are generated for it.

Fixes https://bugzilla.redhat.com/show_bug.cgi?id=2135778.

The code is changed to try to write data opportunistically. If we get partial
information, that is still logged. In is generally better to log partial
backtrace information than nothing at all.