Use a POSIX lock for pkgdb and only issue pkgdb writes in exact points.

- Rather than using a POSIX named semaphore use a POSIX lock (lockf(3))
for pkgdb for writers. Writers that cannot acquire the pkgdb lock will
get EAGAIN rather then being blocked.

- Due to using a file lock we cannot write the pkgdb every time a package
is being unpacked, configured or removed. Instead pkgdb is only written
at the end of a specific point in the transaction (unpack, configure, remove)
or via xbps_pkgdb_unlock().

NEWS

@@ -8,6 +8,9 @@ xbps-0.33 (???):
    the output file will be named "bar.tar.gz". Contributed by
    Enno Boland (Gottox).
 
+ * Rather than performing a pkgdb write per package being unpacked, configured,
+   or removed, issue the write only once after the transaction phase has finished.
+
  * While checking file permissions for package files removal, make sure
    to not dereference symbolic links, because a symlink might point to
    a file in the rootfs, which won't have the same uid/gid.
@@ -20,7 +23,7 @@ xbps-0.33 (???):
 
  * Only allow a single writer to the pkgdb to avoid concurrency issues
    when performing multiple write transactions (install, update or remove).
-   The implementation relies on a POSIX named semaphore.
+   Processes that cannot acquire the lock will get an EAGAIN error.
 
  * Do not continue the transaction if downloading a binpkg/signature file
    has failed for some reason, stop and return error immediately.