tests: use 'hghave execbit' for tests that manipulate x bit in file system

tests: use 'hghave symlink' for tests using symlinks

windows: use umask 022 in debugstate output debugstate would always report files as mode 666 or 777 on Windows. umask is not used on Windows, but faking and using a defalt value of 022 matches what the test suite uses on Unix.

dispatch: exit with 8-bit exit code The exit code returned from a program to the shell is unsigned 8-bit, but Mercurial would sometimes try to exit with negative numbers or None. sys.exit on Unix will convert that to 8-bit exit codes, but on Windows negative values showed up as 0. The exit code is now explicitly converted to unsigned 8-bit.