Fix several places where tool name has been hardcoded:

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.