Return const char * in cgit_{httpscheme, hosturl, rooturl}()

The return values of these functions are essentially constant and should
never be modified.

Note that this will introduce a compiler warning when we try to free the
return value of any of these functions. However, given that all of these
currently return statically allocated strings in some cases, they need
to be refactored before this can be done anyway.

Signed-off-by: Lukas Fleischer <cgit@cryptocrack.de>