Add an option to mandelbrot to make it easier to test parallelism

Add a --parallel command line option to the mandelbrot benchmark that can be
given to use explicitly parallelised routines.  This makes it easier to turn
on and off explicit parallelism, depending on your testing needs.

benchmarks/progs/mandelbrot/mandelbrot.m:
    As above
Do not escape character strings in internal atom representation.

The internal representation of a character constant as an atom term
should not use escaped character syntax; the escaping should occur
when writing the external representation only.

compiler/hlds_out_mode.m:
compiler/intermod.m:
compiler/prog_util.m:
compiler/type_constraints.m:
compiler/typecheck.m:
	Fix instances of the bug.

tests/valid/Mercury.options
tests/valid/Mmakefile
tests/valid/intermod_char.m
tests/valid/intermod_char2.m
	Add test case.  The .opt file was invalid because the character
	in the clause head was doubly-escaped.
Merge fix for bug #330 into version-14_01-branch

Commit 5254245ae34ab6b3ca0f7dcd3c7fc807e1407f7d was inadvertently
merged onto version-14_01-branch but NOT the subsequent fix in
commit 8c72b1e5c0c23353770c494bcdf35a450ebf7c85
Fix no_refs_to_deleted_labels failures.

MR_LOWLEVEL_DEBUG is set when compiling the no_refs_to_deleted_labels
test case to give MR_init_label macros some effect.  However
MR_LOWLEVEL_DEBUG implies MR_DEBUG_GOTOS, and leads to link errors if
the runtime was not built with MR_DEBUG_GOTOS.

tests/hard_coded/Mercury.options:
	Replace MR_LOWLEVEL_DEBUG with MR_DEBUG_LABEL_NAMES,
	implying MR_INSERT_LABELS but not MR_DEBUG_GOTOS.
Merge branch 'version-14_01-branch'
Merge branch 'version-14_01-branch'