MINGW: config.mak.uname: reorganize MINGW settings

HAVE_LIBCHARSET_H and NO_R_TO_GCC_LINKER are not specific to
msysGit, they're general MinGW settings.

Signed-off-by: Marat Radchenko <marat@slonopotamus.org>
MINGW: config.mak.uname allow using CURL for non-msysGit builds

Also, fix `warning: passing argument 2 of 'mingw_main' from
incompatible pointer type` in http-fetch.c and remote-curl.c.

Signed-off-by: Marat Radchenko <marat@slonopotamus.org>