Use read(2) instead of fgets(3) when reading process output.

This patch modifies `tor_read_all_handle()` to use read(2) instead of
fgets(3) when reading the stdout from the child process. This should
eliminate the race condition that can be triggered in the 'slow/util/*'
tests on slower machines running OpenBSD, FreeBSD and HardenedBSD.

See: https://bugs.torproject.org/21654