Fix several places where tool name has been hardcoded:

install -> ${INSTALL}
mtree -> ${MTREE_CMD}
services_mkdb -> ${SERVICES_MKDB_CMD}
cap_mkdb -> ${CAP_MKDB_CMD}
pwd_mkdb -> ${PWD_MKDB_CMD}
kldxref -> ${KLDXREF_CMD}

If you do custom FreeBSD builds you may want to override those
in some cases.

Sponsored by: Sippy Software, Inc.