Do not escape special chars in regex pattern with macro

[martinhsv]

Closes #2357

[marcstern]

Thanks for the fix Martin.
The strdup can be avoided; we can use directly re_pattern->value:
regex = msc_pregcomp_ex(msr->mp, re_pattern->value, PCRE_DOTALL | PCRE_DOLLAR_ENDONLY, &errptr, &erroffset, msc_pcre_match_limit, msc_pcre_match_limit_recursion);

[martinhsv]

Yes, I had noted the possibility of omitting the additional duplication.

However, I opted to leave the pre-existing model in place (by which I mean, to the extent that it creates yet another copy of the string to pass to the regex engine) because of the data models that are in use.

The function expand_macros() populates re_pattern, which is of type msc_string. And in such structures, the 'value_len' can (in principle) be greater than the null-terminated length of 'value'. Also, just thinking from a code-safety perspective, one might wonder whether a code error (now or in the future) might result in an msc_string where the value_len is correct, but the buffer pointed to by 'value', is (incorrectly) not null-terminated (there is no such bug now -- expand_macros() will always correctly null-terminate the returned string, but, still, there can sometimes be merit in safeguarding against some future bug).

Although it makes no functional difference right now, I suspected that it might be slightly cleaner/clearer to logically differentiate the variable as an msc_string from what will be passed to the regex engine. I may revisit that prior to merge.