mgnuc.sh:
Remove support for the -sinit option, since it doesn't work anymore.
Fix typo in -v output.
Add -Wno-unused to the warning options, to suppress warnings
about unused procedures.
mod2c, mgnuc.sh, ml.sh:
Add new grades fast.gc, reg.gc, etc. for conservative garbage
collection. Remove the --gc option from ml, since that information
is now part of the grade.
mgnuc.sh mod2c mod2init.sh:
Moved these scripts from the code directory to the scripts directory.
Changed mgnuc to use the MERCURY_C_INCL_DIR environment variable.
Changed mod2init to use MERCURY_MOD_LIB_DIR and MERCURY_MOD_LIB_MODS.
Changed ml to use MERCURY_C_LIB_DIR rather than MERCURY_CLIB_DIR.