[tex-live] fmttriggers and format rebuilds

[Karl Berry]

FYI all ... I just committed changes that should cause all the formats
to get rebuilt when tonight's update is applied.  It's also not unlikely
that tonight's rebuild will fail (as it did last night), so don't be alarmed.

Details:

This is the implementation of what I alluded to a few days ago, the
method Norbert and I came up with for rebuilding formats when any of the
relevant dependencies change -- a new fmttriggers=p1,p2,... directive in
the *.tlpsrc files and hence in tlpdb.  At the moment, this is done at
granularity of packages, not individual files, but hopefully none of
these dependency packages change gratuitiously often enough to be a
problem.  We'll just see.

The list of "trigger" packages was created by inspecting the --recorder
output from building each .fmt (and the one .base :).  We'll run a
script (tlpkg/bin/check-fmttriggers) from cron to ensure that the lists
remain up to date.

I also made a number of changes to fmtutil itself for better reporting
of error conditions.  These changes are not required for the fmttriggers
stuff, but are just "coincidental".  No changes to normal fmtutil
functionality were intended, though I certainly could have introduced
bugs somewhere.

Anyway, the end result of all this "should" be invisible to normal
installations, but writing it all out here for background since as we
all know, surprises are not surprising.

Good luck to us all,
Karl