log: protect errno in log_open()

Commit 0b1f3c76 has introduced log_open()
calls after exec fails post-fork. However, the log_open() call itself could
change the value of errno, which, for me, manifested in:

$ coredumpctl gdb
...
Failed to invoke gdb: Success

Fix this by using PROTECT_ERRNO in log_open().

(cherry picked from commit 0e557eef)