Revise descriptor stuff.
descriptor.c(h) was broken by the commit
	2da1668f4c7d7871a7212db10af606a7e646abd6
and it now causes a crash bug.
Instead of reverting the change revising the stuff completely.
Remove unsed dialog which was added by mistake.
Remove duplicated line in snprintf() which causes a crash introduced by
the commit 1177281049e8bb3df5992da5dc1ca31ad892082e.
Add a check for NULL dereference pointed out by Pluto Cobain.
It's for safety though I don't think it would occur.
Improve pgxalib.dll(MSDTC support).
1. Change xa_open() so that it tries to connect to the database immediately.
   This would cause an error at enlistment in dtc when the authentication
   method depends not only on the db user but also on the OS user and it
   means the automatic transaction recovery is unavailable.
2. Log the error message in case of SQLDriverConnect() error.
3. Improve the logging.