Win32: keep the environment sorted

Win32: keep the environment sorted

The Windows environment is sorted, keep it that way for O(log n)
environment access.

Change compareenv to compare only the keys, so that it can be used to
find an entry irrespective of the value.

Change lookupenv to binary seach for an entry. Return one's complement of
the insert position if not found (libc's bsearch return's NULL). If a
case-insensitive match is found, try to find a better case-sensitive match
around that entry.

Change do_putenv to insert new entries at the correct position. Simplify
the function by swapping if conditions and using memmove instead of for
loops.

Move qsort from make_environment_block to mingw_startup. We still need to
sort on startup to make sure that the environment is sorted according to
our compareenv function (while Win32 / CreateProcess requires the
environment block to be sorted case-insensitively, CreateProcess currently
doesn't enforce this, and some applications such as bash just don't care).

Signed-off-by: Karsten Blees <blees@dcon.de>