Introduce html.h

All html-functions can be quite easily separated from the rest of cgit, so
lets do it; the only issue was html_filemode which uses some git-defined
macros so the function is moved into ui-shared.c::cgit_print_filemode().

Signed-off-by: Lars Hjemli <hjemli@gmail.com>

shared.c

@@ -14,8 +14,6 @@ int cgit_cmd;
 
 const char *cgit_version = CGIT_VERSION;
 
-int htmlfd = 0;
-
 void cgit_prepare_context(struct cgit_context *ctx)
 {
 	memset(ctx, 0, sizeof(ctx));