ueventd: allow configuring SO_RCVBUF(FORCE) for the ueventd socket

Some configurations won't allow ueventd to have CAP_NET_ADMIN, so the
new default size of 16M is not possible for those.  Those
configurations also won't need such a large buffer size, so this
change allows devices to customize the SO_RCVBUF(FORCE) size for the
uevent socket.

This is done by adding the line 'uevent_socket_rcvbuf_size <size>' to
your device's ueventd.rc file.  <size> is specified as a byte count,
for example '16M' is 16MiB.

The last parsed uevent_socket_rcvbuf_size line is the one that is
used.

Bug: 120485624
Test: boot sailfish
Test: ueventd unit tests
Change-Id: If8123b92ca8a9b089ad50318caada2f21bc94707
diff --git a/init/ueventd_parser.h b/init/ueventd_parser.h
index 7d30edf..d476dec 100644
--- a/init/ueventd_parser.h
+++ b/init/ueventd_parser.h
@@ -31,6 +31,7 @@
     std::vector<Permissions> dev_permissions;
     std::vector<std::string> firmware_directories;
     bool enable_modalias_handling = false;
+    size_t uevent_socket_rcvbuf_size = 0;
 };
 
 UeventdConfiguration ParseConfig(const std::vector<std::string>& configs);