Remove imp.h and exp.h header files

These include files don't have a function any more, remove them.
io.c: <sys/time.h> is only needed when using select()

Only include the <sys/time.h> header when using the select() IO API,
it isn't required otherwise.
configure[.ng]: <sys/types.h> is a required header file

ngIRCd already includes <sys/types.h> in a lot of places without
checking for its existence (for example in "ngircd.c", "io.c", ...),
therefore make it a required header file.