Introduce para_regcomp.

A wrapper for regcomp() that logs an error message if the regcomp() failed
and uses a return value according to the paraslash rules. Currently there is
only one user of regular expressions, is_v4_dot_quad(), which is converted
to para_regex(), but new callers will be added soon.

Unfortunately, this change made it necessary to include regex.h in all .c files
that use string.h. Clean up the order in which headers are included a bit while
we're at it.

Maybe I should rethink the rule "Only .c files shall include header files"...