Make sure the kernel lock is held when invoking process_domem(); fixes
ptrace operation on MP kernels.
In fpu_compare(), set the `not equal' bit when the result of the comparison
is `not comparable'. gcc relies upon `not equal' being set when comparing
numbers to infinities.
Sync description of PROT_* flags between mmap.2 and mprotect.2

ok guenther
When .Sm is called without an argument, groff toggles the spacing mode,
so let us do the same for compatibility.  Using this feature is of
course not recommended except in manual page obfuscation contests.
Improve and test the messages about empty macros,
in particular reporting the macro names involved.