write unified chain loader
  - arbitrary mapping
  - enable/disable clauses
    [disk|part|field|old|new|mask]
update tech.tex
known bugs: jmpi #GO, FIRSTSEG may not work
better errors about bad version
support "simple" bootdisks
describe how to build a boot floppy
check tech.tex for obsolete parts
tex2nroff for man page
build an index for user.tex
specify need to set rewrite-table for fix_table, etc.
automate XL_SECS
