As a general design principle, strings that we aren't going to modify
should usually be const. When compiling with -Wwrite-strings, quoted
string constants are of type "const char *", causing compiler warnings
when they are assigned to char * variables.

Unfortunately, we need to add casts in a few places:

* _dbus_list_append(), _dbus_test_oom_handling() and similar generic
  "user-data" APIs take a void *, not a const void *, so we have
  to cast
* For historical reasons the execve() family of functions take a
  (char * const *), i.e. a constant pointer to an array of mutable
  strings, so again we have to cast
* _dbus_spawn_async_with_babysitter similarly takes a char **,
  although we can make it a little more const-correct by making it
  take (char * const *) like execve() does

This also incorporates a subsequent patch by Thomas Zimmermann to
put various string constants in static storage, which is a little
more efficient.

Signed-off-by: Simon McVittie <smcv@debian.org>
Reviewed-by: Thomas Zimmermann <tdz@users.sourceforge.net>
Bug: https://bugs.freedesktop.org/show_bug.cgi?id=97357