Read Holmakefiles fewer times, speeding up startup times

This speeds up Holmake, hol and script files.

Also remove output of loadPath deltas as hol starts.

Make Holmake's internal_functions' find_unescaped more efficient

Profiling showed this function taking rather longer than one would
want.

Fix Holmake's wildcard fn to interpret arg as multiple patterns

So, $(wildcard foo* bar*) now searches current directory for files
beginning with foo, and those beginning with bar. Previously, the
space would have been interpreted as a literal space. To get the
literal space, you need to escape it with a backslash (as per the 8th
testcase in the regression test directory).

Fix Holmake's $(wildcard ...) fn to return "" when matches fail

This is consistent with documentation at

https://www.gnu.org/software/make/manual/html_node/Wildcard-Function.html

except that suggests you can have multiple wildcard patterns rather
than just one. (Just noticed this!)

Thanks to Magnus for discussion leading to fix.

Replace "failure" message with an empty string

Remove carriage returns from input

Add support for 'shell' in Holmake

Implement & document which fn for Holmakefiles

Implement wildcard function for Holmake.

Closes #36

Add new findstring function to Holmake language (copying GNU make).

Start to implement the Basis 97 extensions that Moscow ML hasn't got
in order to force our codebase to get up-to-date. It should also
mean less bodging around for the Poly/ML code. I haven't checked that
my changes to tools-poly/poly/poly-init2.ML have done all that is
required yet. Feel free to fix problems arising there (I hope it will
just be a matter of deleting things).

Introduce new makefile function called dprot for quoting file-names that
may include spaces and which need to be mentioned as dependencies in
Holmakefiles. Document its existence in the Description. Tighten up
some of the text-processing done inside Holmakefiles.

Fix for a problem Konrad identified with the way that Holmake was protecting
its built-in variables (MOSMLCOMP etc). Now Holmake doesn't protect
variables that may be used in situations where they are concatenated
with others, but does protect those that will be used on their own (such
as MOSMLC etc).
This requires the provision of a protect function in make-files, so
this is documented in the DESCRIPTION. I think things should be right,
but please check that this works, particularly anyone on Windows!

Implement patsubst internal function, and shift some useful code into
a separate file.