Fix concurrency issues in pkgdb: only allow 1 write transaction at the same time.

This implementation relies on a POSIX named semaphore, which is also
required by xbps-rindex(8).

README.md

@@ -35,6 +35,7 @@ See `git tag -l` to list all available stable releases.
 To build this you'll need:
 
   - A C99 compiler (clang and gcc tested)
+  - A system that supports POSIX named semaphores
   - [GNU make](http://www.gnu.org/software/make/)
   - [pkg-config](http://www.freedesktop.org/wiki/Software/pkg-config/)
   - [zlib](http://www.zlib.net)