exterr: relax format restrictions

Rather than passing the format string to printf and forcing the
arguments to be (u)intmax_t, partially parse format strings and cast
p1 and p2 to the correct type before running the individual format
though printf.  This restructure has a couple motivatations:
 - We can skip formats that make no sense (floating point, %n, etc.).
 - It is possible to special case the printing of pointers in the
   CHERI case.

The first case is motivated by a suggestion from the audiance at
one of Kirk's BSDCan talks on exterr to allow userspace to set exterr
status.  Allowing arbitrary format strings including %n creates a
write-what-where gadget so we need to not do that.

The second case is motivated by our experinces with CHERI and debugging
mmap issues using a different textual error reporting framework.  With
CHERI, pointers are more than integer addresses and it's useful to
include more details.  Doing so will follow in a future commit.

When the new code encounters an inappropriate format it includes
a diagnostic and in most cases prints the format untouched.

Reviewed by:	kib
Effort:		CHERI upstreaming
Sponsored by:	Innovate UK
Differential Revision:	https://reviews.freebsd.org/D58058

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