libc: configurable initial FDs

The libc already supports the configuration of 'stdin', 'stdout', and
'stderr' using '<libc>' config attributes. This patch equips the libc
with the additional ability to pre-initialize any other file descriptor.
A file descriptor is configured as follows:

<config>
  ...
  <libc ...>
    <fd id="3" path="/dev/log" writeable="yes" readable="no" seek="10"/>
    ...
  </libc>
</config>

Furthermore, this patch moves the FD initialization code from the VFS
plugin to the libc kernel initialization because opening the FDs
depends on 'malloc' ('strdup'), which should not be used at early
'Libc::Kernel' initialization time.

Issue #3478

repos/libports/include/libc-plugin/fd_alloc.h

@@ -109,6 +109,11 @@ namespace Libc {
 			 */
 			void free(File_descriptor *fdo);
 
+			/**
+			 * Prevent the use of the specified file descriptor
+			 */
+			void preserve(int libc_fd);
+
 			File_descriptor *find_by_libc_fd(int libc_fd);
 	};