exewrapper: add .dll to LoadLibrary() argument stable

exewrapper: add .dll to LoadLibrary() argument

LoadLibrary() changes behavior depending on whether the argument
passed to it contains a period. From the MSDN docs:

If no file name extension is specified in the lpFileName parameter,
the default library extension .dll is appended. However, the file name
string can include a trailing point character (.) to indicate that the
module name has no extension. When no path is specified, the function
searches for loaded modules whose base name matches the base name of
the module to be loaded. If the name matches, the load succeeds.
Otherwise, the function searches for the file.

As the subsequent patch will show, some environments on Windows
define their Python library as e.g. "libpython2.7.dll." The existing
code would pass "libpython2.7" into LoadLibrary(). It would assume
"7" was the file extension and look for a "libpython2.dll" to load.

By passing ".dll" into LoadLibrary(), we force it to search for the
exact basename we want, even if it contains a period.

--- a/mercurial/exewrapper.c	Wed Apr 27 14:02:54 2016 -0700
+++ b/mercurial/exewrapper.c	Wed Apr 27 09:23:39 2016 -0700
@@ -96,7 +96,8 @@
 			/* path pyhome exists, let's use it */
 			FindClose(hfind);
 			strcpy_s(pydllfile, sizeof(pydllfile), pyhome);
-			strcat_s(pydllfile, sizeof(pydllfile), "\\" HGPYTHONLIB);
+			strcat_s(pydllfile, sizeof(pydllfile),
+				 "\\" HGPYTHONLIB ".dll");
 			pydll = LoadLibrary(pydllfile);
 			if (pydll == NULL) {
 				err = "failed to load private Python DLL "
@@ -114,7 +115,7 @@
 	}
 
 	if (pydll == NULL) {
-		pydll = LoadLibrary(HGPYTHONLIB);
+		pydll = LoadLibrary(HGPYTHONLIB ".dll");
 		if (pydll == NULL) {
 			err = "failed to load Python DLL " HGPYTHONLIB ".dll";
 			goto bail;