log: protect errno in log_open()

log: protect errno in log_open()

Commit 0b1f3c768ce1bd1490a5e53f539976dcef8ca765 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 0e557eef37c9ebcc8f5c19fc6fc44b6fd617cc5d)
(cherry picked from commit dabf9f2109e5b8cc26d9e0d9188deebff3edf8c2)