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.
exterror: Add EXTERROR_VERBOSE env variable to control verbosity
If the variable is set and the process is not suid, __uexterr_format(),
used by err(3), prints errno/category/source line/pX always, not only
when there is no kernel message provided.
vm/vm_mmap.c: inline erronous argument values for extended errors