Add support for 'dmesg -c' which clears the dmesg buffer after it has
been printed.  This provides compatibility with other *nix systems
(including Linux).

While here use stdbool booleans for 'all'.

PR:		bin/178295
Submitted by:	Levent Serinol <lserinol@gmail.com>
Reviewed by:	will
Style(9) changes fo xargs.c