Finally extract the log library and make it build.

This patch:
  - introduces an fdio module for low-level fd functions that don't
    need to log.
  - moves the responsibility for opening files outside of torlog.c,
    so it won't need to call tor_open_cloexec.

Makefile.am

@@ -42,6 +42,7 @@ TOR_UTIL_LIBS = \
 	src/common/libor.a \
         src/lib/libtor-log.a \
         src/lib/libtor-lock.a \
+        src/lib/libtor-fdio.a \
 	src/lib/libtor-container.a \
         src/lib/libtor-string.a \
 	src/lib/libtor-malloc.a \
@@ -56,6 +57,7 @@ TOR_UTIL_TESTING_LIBS = \
 	src/common/libor-testing.a \
         src/lib/libtor-log-testing.a \
         src/lib/libtor-lock-testing.a \
+        src/lib/libtor-fdio-testing.a \
 	src/lib/libtor-container-testing.a \
         src/lib/libtor-string-testing.a \
 	src/lib/libtor-malloc-testing.a \