exterror: support %d in error messages

[kib]

Summary

libc/gen/err.c: remove 'extended error' herald from extended error output


exterror: add support for the format specifiers in the extended error msg

Note that we trust kernel code to only request the printout of integer
types, and use the 'j' modifier always.

vm/vm_mmap.c: inline erronous argument values for extended errors

[kib]

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