Bug in timer{add,sub} compatibility functions
```
< Vektor> i have another bugfix for tor
< Vektor> replace:
< Vektor> #define timeradd(tv1,tv2,tvout) \
< Vektor> do { \
< Vektor> (tvout)->tv_sec = (tv1)->tv_sec + (tv2)->tv_sec; \
< Vektor> (tvout)->tv_usec = (tv2)->tv_usec + (tv2)->tv_usec; \
< Vektor> with:
< Vektor> #define timeradd(tv1,tv2,tvout) \
< Vektor> do { \
< Vektor> (tvout)->tv_sec = (tv1)->tv_sec + (tv2)->tv_sec; \
< Vektor> (tvout)->tv_usec = (tv1)->tv_usec + (tv2)->tv_usec; \
< Vektor> and replace:
< Vektor> #define timersub(tv1,tv2,tvout) \
< Vektor> do { \
< Vektor> (tvout)->tv_sec = (tv1)->tv_sec - (tv2)->tv_sec; \
< Vektor> (tvout)->tv_usec = (tv2)->tv_usec - (tv2)->tv_usec; \
< Vektor> with:
< Vektor> #define timersub(tv1,tv2,tvout) \
< Vektor> do { \
< Vektor> (tvout)->tv_sec = (tv1)->tv_sec - (tv2)->tv_sec; \
< Vektor> (tvout)->tv_usec = (tv1)->tv_usec - (tv2)->tv_usec; \
```
These definitions are only used in platforms where `timersub` and `timeradd` are not defined.
`timeradd` doesn't seem to be used anywhere in the code, and `timersub` only seems to be used in `circuit_expire_building()`.
At first glance, the issue doesn't seem security related, but we should look at it carefully.
issue