Create and use struct_sysent and struct_ioctlent typedefs.

Create and use struct_sysent and struct_ioctlent typedefs.

This is a preparatory mass replace patch with no code changes.

The future change will need to typedef sysent to sysent0,
which results in compile failures when "struct sysent" string
gets mangled into "struct sysent0".

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>