make omake UTF-8 aware

[gerdstolpmann]

Up to 0.10.2 omake always assumes the Latin-1 character set, even when dealing with external strings like file names. As of 2017, it is more reasonable to always assume Unicode.

In 0.10.3, the Latin-1 support is removed. Where assumptions about the charset are made, only ASCII is supported. E.g. we can only convert between uppercase and lowercase for ASCII.

OCaml 4.06 support now also UTF-8 filenames on Windows. (In other words, assuming the local code page is now wrong.)

We need:

• UTF-8 access (basic string functions) and validation
• conversion between uppercase and lowercase (a conversion table can be found here: https://gitlab.camlcity.org/gerd/lib-ocamlnet3/blob/master/code/src/netstring/unicode_charinfo.txt)
• Win32: use the "W" functions (fam_win32.c (monitor_directory), lm_unix_cutil.c (home_win32), omake_shell_sys (calls CreateProcess which is also affected)