When implementing custom log_... functions or custom overloads for the
core log functions like log_signal it is necessary to return `char *`
that are valid long enough.
The log_... functions implemented in log.cc use either `log_id_cache` or
`string_buf` which both are cleared on log_pop.
This commit adds a public `log_str` function which stores its argument
in the `log_id_cache` and returns the stored copy, such that custom
log functions outside of log.cc can also create strings that remain
valid until the next `log_pop`.
c73c8a39cf