add some errhint for regexp* functions.

hi.

Some of the regexp* function error messages seem not very helpful, I think.
like:
ereport(ERROR,
errcode(ERRCODE_INVALID_PARAMETER_VALUE),
errmsg("invalid value for parameter \"%s\": %d",
"start", start));

so I added a errhint, errhint would be better than refactoring existing errmsg.
for the above example, I've add:

+ errhint("\"%s\" must be an integer value greater than zero", "start")));

--
jian
https://www.enterprisedb.com