This is a non-POSIX/ISO-C header. It is ok to include this on Linux, but it
is obsolete on BSD; FreeBSD even throws an error if you include it with
__STDC__ defined. Every system should nowadays have malloc() defined in
stdlib.h.
Diff is largely mechanical, replacing malloc.h with stdlib.h where it is not
yet included anyway.
Remove from individual Imakefiles.
Also, remove '#if 0' block in linux.cf, and remove empty
LinuxMachineDefines. This should be working correctly. If not, let me
know.