Avoid more C compiler warnings in the library and compiler directories.

library/io.m:
	Add casts where appropriate to avoid warnings.

	Return a dummy time value in io.file_modification_time_2/6 when the
	call to stat() fails.  (This is safe since the caller will never
	examine this value if the call to stat() fails.)  Doing so avoids
	a warning about the time value not being initialized in some branches.

library/string.m:
	Delete unused local variables in foreign_procs.

library/time.m:
compiler/timestamp.m:
	Cast assignments to the fields of the tm structure to ints.

compiler/md4.m:
	Avoid a warning about comparing integers of different signedness.

compiler/process_util.m:
	Add casts where appropriate to avoid warnings.