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.