Rename a large chunk of the make(1) variables which refer to a

Rename a large chunk of the make(1) variables which refer to a
program/tool from "FOO" to "TOOL_FOO".  The new variables are:
	TOOL_ASN1_COMPILE  TOOL_CAP_MKDB  TOOL_CAT  TOOL_CKSUM  TOOL_COMPILE_ET
	TOOL_CONFIG  TOOL_CRUNCHGEN  TOOL_CTAGS  TOOL_DB  TOOL_EQN  TOOL_FGEN
	TOOL_GENCAT  TOOL_GROFF  TOOL_HEXDUMP  TOOL_INDXBIB  TOOL_INSTALLBOOT
	TOOL_INSTALL_INFO  TOOL_M4  TOOL_MAKEFS  TOOL_MAKEINFO  TOOL_MAKEWHATIS
	TOOL_MDSETIMAGE  TOOL_MENUC  TOOL_MKCSMAPPER  TOOL_MKESDB
	TOOL_MKLOCALE  TOOL_MKMAGIC  TOOL_MKTEMP  TOOL_MSGC  TOOL_MTREE
	TOOL_PAX  TOOL_PIC  TOOL_PREPMKBOOTIMAGE  TOOL_PWD_MKDB  TOOL_REFER
	TOOL_ROFF_ASCII  TOOL_ROFF_DVI  TOOL_ROFF_HTML  TOOL_ROFF_PS
	TOOL_ROFF_RAW  TOOL_RPCGEN  TOOL_SOELIM  TOOL_SUNLABEL  TOOL_TBL
	TOOL_UUDECODE  TOOL_VGRIND  TOOL_ZIC

For each, provide default in <bsd.sys.mk> of the form:
	TOOL_FOO?=	foo
and for the ${USETOOLS}=="yes" case in <bsd.own.mk>, provide override:
	TOOL_FOO=	${TOOLDIR}/bin/${_TOOL_PREFIX}foo

Document all of these in bsd.README.

This cleans up a chunk of potential (and actual) namespace collision
within our build infrastructure, as well as improves consistency in
the share/mk documentation and provision of appropriate defaults for
each of these variables.