[tex-live] mktexfmt -> fmtutil-user/sys

[Akira Kakuto]

> * mktexfmt runs "user mode" of fmtutil on unix (r43793)
> * mktexfmt.exe on win32 runs in "system mode" (r44449)

In the case of windows, it is expected that fmtutil-sys
may not be rejected if TeX Live is installed under a
suitable directory. Thus I decided to choose fmtutil-sys.

Anyway it must me quite rare that mktxfmt is called.

Best,
Akira